Fire Opal, Opal, Opals, Black Opal, Fire Opal, Boulder Opal, Opal Rough, Opal Black Doublet, Blue Opal, Australian Opal, Peru Opal, Cherry Opal, Opal Doublet, Jelly Opal, Multi Color Opal, Orange Opal, Pink Opal, Red Opal, Semi Black Opal, Yellow Opal,Mexican fire opal Opal Black australian opals,boulder opal,black opal,opal gemstones,opal jewelry,fire opal,precious opal,opals,red,white opal,crystal opal,australian black opal,natural,opal gemstones,natural opal,australian opals,boulder opal,black opal,opal gemstones,opal jewelry,fire opal,precious opal,opals,red,white opal,crystal opal,australian black opal,natural,opal gemstones,natural opals Did you know that 95% of the world's precious opal comes from Australia? How Opal Color is Produced It took the development of the electron microscope to work this out. Precious opal is made up of tiny uniform spheres of transparent hard silica, which fit together in an orderly three dimensional frame, sitting in a "bath" of silica solution. It is the orderliness of the spheres that separates precious opal from common opal. Light passes through the transparent spheres in a direct line, but when it hits the 'bath' of silica, it is bent and deflected at different angles, thus producing a rainbow effect. Deflection & Diffraction Depending on the size of the spheres, varying colors of the spectrum are diffracted. So it is a combination of deflection (bending) and diffraction (breaking up) of light rays that creates the color in opal. If you move the stone, light hits the spheres from different angles and bring about a change in color. The name opal actually means "to see a change in color." The way in which colors change within a particular stone as it is rotated and tilted is called the stone's play of color. How color is defined The size of the spheres has a bearing on the color produced. Smaller spheres bring out the blues, from one end of the spectrum. Larger spheres produce the reds from the other end. The more uniform the spheres are placed, the more intense, brilliant and defined the color will be. How Opal Formed The consensus among geologists is that almost all Australian gem opal has formed by precipitation of silica from very dilute silica solutions or colloidal suspensions, derived from the deep weathering of feldspathic sedimentary rocks under the action of percolating groundwater. This deep weathering is a chemical alteration. The feldspar minerals are altered to kaolinite, releasing silica in an aqueous solution or suspension which may collect in traps or cavities in the rocks. These cavities may be open fissures, interstices between particles in a conglomerate, holes left by dissolution of shells, bones, wood or inorganic minerals (such as gypsum and calcite), or hollow cores and cracks in ironstone concretions. The ironstone concretions may be localised along the base of palaeochannels, on the downthrown side of differential compaction faults, or along basal undulations of bedding interfaces. The silica solution or suspension may have been concentrated by evaporation through the overlying sediments, or concentration may have been effected by the action of clay beds which in many places underlie the opal-bearing horizons, as semi-permeable membranes allowing the water to pass through but retaining the silica. Increased concentration my have caused the separation of discrete silica particles which aggregated by collision into equidimensional spheres. When these reached a particular size, they may have undergone undisturbed settling and hardening into an ordered arrangement which formed precious opal. More commonly, a disordered arrangement of spheres accumulated during settling forming common opal or 'potch'. Because water is retained in the opal structure, application of heat, or in some cases merely exposure to a dry atmosphere, can cause fractures to develop. Opaline silica is common throughout the leached profile of the Cretaceous Winton Formation , which consists of feldspathic sandstone, siltstone, and mudstone. In some places, the matrix of the sandstone is replaced by opaline silica, which may also replace wood and gypsum. Precious opal, though sometimes found in these modes, is most commonly associated with various forms of concretionary ironstone. Glossary of Terms Andamooka Matrix - A more porous opal found in the mining field of Andamooka. When it comes out of the ground it is quite pale but by treating it with a carbon dye process it eventually looks like real black opal. An Andamooka matrix opal can be an affordable alternative to a genuine black opal--but buyers beware--an honest opal dealer will tell you whether you are buying an Andamooka matrix or a genuine black; a disreputable dealer may not. Amorphous - Shapeless. Not consisting of crystals. Non crystalline. Glass is amorphous. Sugar is crystalline. Black opal - Opal which is found with a natural black or dark background. This background can range from pitch black to gray giving the stone are darkish appearance when seen from the top. It is the dark background which allows the brilliant colors. Black opal can be any color. Black Crystal Opal - Is gem colors on a smoky, transparent background, some people regard this as the best type of opal ever found. Body Color - This is the background color of the opal (not the color of the "flashes"); and is described as being from white, to gray, to black. Boulder opal - Similar to black opal except that the foreground color is very thin and sits on a brown ironstone base. These stones at times can come with interesting "hills" and "valleys" on the surface. (meaning that the surface is often, but not always, undulating) These stones are preferred by people who are more "progressive" in their jewelry tastes. Often boulder opal has ironstone inclusions in the foreground and all sorts of odd shapes which makes them a designers delight. Broken Flash - Part of the stone displays a flash of color at a time. Consistency of Fire - This has to do with the stone's ability to show play of color when viewed from any angle. In other words, the fire is non-directional. The opposite, directional, indicates that fire is visible only when viewed from a certain angle, and are less valuable. Crystal opal - An opal with a brilliant crystal appearance allowing you to look down into the stone. Deflection - The bending of rays of light from a straight line. Diffraction - The Breaking up of a ray of light into either a series of light and dark bands, or into colored bands of the spectrum. Diffuse - To spread out so as to cover a larger space or surface. To scatter. Fiery Opal - Predominantly red pay of color sometimes seen as pin fire or stardust flashes. Floral - A flashy effect like a bunch of flowers, seen frequently in Queensland opal. Fluorescent - A light produced by the electrical stimulation of a gas or vapor. Fluorescent lights have a similar effect on opal as a bright cloudy day--they do not properly bring out the colors in opal. Gem/Precious Opal - Is defined by the brilliance of colors in the color bar. If it is on flawless potch (common opal), it is considered more valuable than on a mixed layer of potch. Hardness - Opal is 5.5 to 6.5 or harder, similar to Emerald. Harlequin - Is surely at the top of the list and the most prized of all brilliant opals. Such rarities portray spangles of rounded, angular to roughly square patches, presenting a harlequin appearance of interchanging colors. Hydrate - A compound produced when certain substances chemically combine with water. Hue/Fire Pattern - This describes the geometrical relationships of the colors. Mosaic (a "tiled" look), pinfire (lots of small/tiny spots of color), and flash (broad, sweeping, 3-dimensional color swaths that depend on the angle of viewing) are typical terms. Hues and Intensity - The predominant and secondary hues are described for the play of color (flashes) present, along with a percentage of the stone "covered" by each color, and each color's intensity from poor to excellent. Other hues (tertiary, etc.) present may be listed if significant. Jelly Crystal opal - A solid crystal opal that is extremely translucent, to the point of being almost transparent. Incandescent - Glowing with heat (red or white hot) as in a light bulb which glows white hot, but produces a light that more closely simulates natural sunlight. Sunlight and incandescent lights bring out the natural colors in opal. Matrix Boulder Opal - A porous sedimentary ironstone having minute cavities filled with precious opal. When cabochon cut, the dark brown ironstone sparkles with vivid pin flecks of color Mosaic Opal. The previously described opals are all naturally formed. The mosaic opal is a man-made creation. Slivers of genuine opal are assembled into an irregular tiled pattern with rivers of black potch framing each unique opal tile. Mosaic opals make stunning stones for all types of jewelry and allow the jeweler to include a wide range of colors within an affordable piece. Now, there is a natural stone that looks very similar to mosaic opal. The natural phenomenon is called the harlequin opal. It is the rarest and most expensive opal. Opal - Opal comes from the Latin word opalus which means to see a change in color. Chemically, opal is hydrated silica, similar to quartz. Specs: Color: white to gray to black, blue, green, orange Hardness: 5.5 to 6.5 Specific Gravity: 2.0 to 2.2 Cleavage: none Fracture: conchoidal, splintery, brittle Refractive Index: about 1.45 Transparency: transparent to opaque Chemical Composition: SiO2 . nH2O Opalescence - A play of color, similar to that of an opal. Opal fossils - Opalized shells, crustations, sea creatures, snails, animal claws and bones, are some of the many objects that over time can opalize when buried in the unique environment of Australian clay and left undisturbed. Opalized Wood - Because the creation of opal is a natural process, opals can sometimes be found within substances other than stone. Opal patterns - May be classified as follows: pinfire (small pinpoints of color), flashfire (larger irregular pattern of color), broadflash (sheets of color across large portions of the stone), rolling flashfire (sheets of color that "roll" across the stone as it is moved), and harlequin (square/angular blocks of color set closely together; rare). Opaque - Not allowing light to pass through. The opposite of transparent. Peacock - Resembling the tail of a peacock. Radiating from one central point and fanning out like the tail. Picture Opals - Uniquely patterned boulder opals where an object is clearly visible. The specimen has been named "the shrimp." Picture opals are both rare and unique, making them much sought after by opal collectors. Pin Fire - Seen as pinhead spots in one predominant color such as green. The most outstanding being a color unit, perhaps red, radiating one way, which changes to another, perhaps green, when turned at various angles. Some pin fire colors blend into peacock tail design either on part of the stone or from a central nucleus. Other pin fire colors blend into fern or foliage design. Play of Color - The way in which colors change as an opal is tilted in different directions. Play of color in an opal, (its "flashes" or "fire") is caused by light interference patterns created by the tiny spheres that make up opal's structure. These spheres are about 0.001 millimeters in diameter! Potch (Common Opal) - Common opal with no play of color. Varies from black to milk white to greenish, yellowish or bluish in color. Rainbow Opal - The colors are positioned as in a rainbow. Rolling Flash - Brilliant colors rolling across the face of opal, some having cat's eye rolling effect. Semi-Black opal - The semi-black opal is found on most fields. It's background color ranges from gray to near black. One of its distinguishing characteristics is an almost smoky appearance. Semi-black opals are from the same family as black opals, but they are not as dark. Silica - (Silicon Dioxide) A hard, white or colorless substance, that in the form of quartz, enters into the composition of many rocks and is contained in sponges and certain plants. The needle in the mouth of a female mosquito is made of silica. Flint, sand, chalcedony, and opal are examples of silica in different forms. Specimen - A chunk of opal that is either left in its natural state or given a polish or varnish to show off one aspect or feature of the stone. Opal specimens are sought after by rock collectors, who generally display them on a desk or shelf. Spectrum - The band of colors formed when a beam of white light passes through a prism or by some other means (e.g. mist or spray, in the case of a rainbow) The full range of spectrum colors are: red, orange, yellow, green, blue, indigo, and violet. Sphere - A round three dimensional geometric shape whose surface is equally distant at all points from the center point. Translucent - Letting light through without being transparent. Transparency - The "see-throughness" of the body is described (from watery to opaque). Transparent - Easily seen through (glass like) White opal - A solid opal with an opaque light background. Yowah Nut - Brilliantly colored opals nested in a nugget of ironstone. Though the opal center is sometimes separated from its ironstone nugget, many beautiful and unusual jewelry pieces are made by keeping the nugget and its center-opal together. opal, about opal, opal information, opal facts, learn about opal, color grade, clarity, aniversary gemstones, birthstone, gem,cut, order, secure, inventory, sale, free, price, shape, cut, information, opal, lot, rough, synthetic, natural, man made, lab, simulant About Opal History Opal: Nature's Fireworks Mysterious opals contain the wonders of the skies - sparking rainbows, fireworks, and lightning - shifting and moving in their depths. Opal has been treasured throughout history around the world. Archaeologist Louis Leakey found six-thousand year old opal artifacts in a cave in Kenya! Roman historian Pliny described the beauty of opal as the combination of the beauty of all other gems: "There is in them a softer fire than the ruby, there is the brilliant purple of the amethyst, and the sea green of the emerald - all shining together in incredible union. Some by their splendor rival the colors of the painters, others the flame of burning sulphur or of fire quickened by oil." Opal was much loved and valued highly by the Romans, who called it opalus. At the same time, opal was also sought in what would become the Americas. The Aztecs mined opal in South and Central America. About Opal CHOOSING THE RIGHT OPAL When you are choosing your opal it is VITAL that you know how the opal will be worn, as some opal will look brighter when viewed vertically whereas others will show more colour when viewed horizontally. SOLID OPAL Opal that has been mined and is presented either in its naturally occurring state or after being cut and polished. It has not been chemicalIy treated and has no other materials cemented to it. Opal is made up of close packed aggregates of silica spheres, and with a water content between 3-10%. In precious opal the arrangement of spheres is in orderly layers, and light passing through the spheres is diffracted at the void and layer interface to produce the vivid play of colour associated with opal. Larger silica pheres are associated with more sought after colours, such as red. HARDNESS It has hardness ranging between 5-6.5, is brittle with a conchoidal fracture and some light varieties fluoresce white or yellow under long and short wave ultraviolet light. This property is used on South Australian fields such as Coober Pedy to noodle opal from tailings, which are run on conveyor belts through darkened sheds, past UV lighting. BLACK OPAL Is generally more sought after and more valuable than light opal. In Australia the major opal producing fields for black opal are Lightning Ridge in New South Wales and Mintabie in South Australia. Is usually found as nobbies - small blocks, pillows, spheres or hat shaped stones ranging from around 1-5 cm across. The stones usually have light grey appearance when found due to a thin outer layer of grey potch. When stones are clipped they reveal black potch inside along with any colour bar. Opal from Lightning Ridge is often considered to be the best and brightest in the world BLACK JELLY CRYSTAL From the Lightning Ridge field. Of the same family as black opal without the black potch background. BOULDER OPAL Opal that has formed naturally on ironstone or "boulder" which is cut to form that host rock. DOUBLETTES Some of the world’s most beautiful opals are too thin to use alone. If they are glued onto a backing material for strength, they are called "Doublets". Sometimes they will have a clear quartz cap put on them for even more security. Then they are called "Triplets". A veneer of precious opal is cemented to a dark or black base to provide sufficient depth to the stone to enable it to be set in jewellery and enhance its natural colour. WHITE OPAL A predominantly white or opaque background colour in refractive plane. Imitation white opals are uncommon. There are some plastic or glass imitations but, owing to the low cost of commercial-grade opals, these are not usually seen. There are also lab-grown opals but these can be usually discerned under magnification. CAUTIONS Opal is beautiful but also delicate. It should never be put in an ultrasonic machine or a steamer. Sudden changes in temperature or a sharp blow can cause an opal to crack or fracture. Opals have a high water content and can therefore dry out. This can also cause cracking. To a large extent, a given stone’s susceptibility to cracking is related to the quality of the rough material from which it was cut. Certain mines are known for producing better-quality material than other mines. It is possible for two opals to look almost exactly the same and have one crack in a few months and the other last for years. TREATMENTS It is common for white opals to be impregnated with oil or wax to prevent cracking. The efficacy of this treatment is questionable because it is not permanent. Sometimes these and other fillers can be used to repair opals that have already cracked. In such cases, disclosure is imperative. TYPE OF OPAL BLACK OPAL A solid Opal which is opaque when viewed from the top of the stone, and which has a play of color against a dark background. The back of the Opal may be any color. BLACK CRYSTAL A solid Opal which is translucent to transparent which has a play of color against a dark gray background. BOLDER BLACK A natural bolder Opal which has a play of color against a dark opaque background when viewed from the face. BOLDER OPAL An Opal that is still attached to the host matrix, usually ironstone, in a seam or in patches. BOLDER MATRIX A combination of Opal and ironstone where the Opal is mixed through the ironstone rather than in seams. BOLDER DOUBLET A two part assembled Opal made of precious Opal with an ironstone or other backing glued to it. It can be natural or put together by man. SEMI-BLACK A solid Opal which is translucent to opaque when viewed from the top and which has a play of color against a dark gray background. CRYSTAL OPAL A solid Opal which is transparent showing a play of color and no base color. This Opal will show little color when on a white background. SEMI-CRYSTAL A solid Opal which is translucent showing a play of color and a clear to slightly gray or white base. WHITE OPAL A solid Opal which is opaque or translucent showing a play of color on a white to off white base color. PRECIOUS OPAL Opal which displays a play of color in a distinct pattern. COMMON OPAL Opal which does not show a play of color or some general opalescence only. Potch. JELLY OPAL A solid Opal which is transparent showing no play of color. It may show some Opalescence without a pattern. FIRE OPAL A solid Opal with a transparent orange to red-orange base color typically faceted from Mexico. GRAY OPAL A solid Opal which is opaque or translucent showing a play of color on a gray base. BASE COLOR: Determined by looking through the top of the Opal beyond the play of color. CUT: Not important unless the Opal is thin and very breakable, or straight high cabochon , which would be hard to set. If the shape is appealing to you - go for it. CLARITY: Opals must have inclusions in order to have color. Avoid gray, cotton or white areas that are dull and show no color. Always put a penlight under Opals and stay away from any that have gray dried out areas and/or cracks. COLOR: Color is almost everything in Opals. The more color the better. Reds, pink, yellow and orange are harder to find, and therefore are more expensive. Green, blue and violet are much more prevalent. PATTERN VARIETY: Harlequin (mosaic), Pinfire, Flash, Flame, Flagstone, Ribbon, Peacock, Rolling Flash and many, many others. No one pattern is more expensive than another. It's the amount of color and its intensity that makes it better than another. BRIGHTNESS OF FIRE LEVEL NAME DESCRIPTION 1 Faint Shows a play of color only under direct sunlight, and even then, the fire is faint or almost non-existent. 2 Dull Shows some color under low light, but is dull even under indirect sunlight or the grading lamp. 3 Bright Shows fair color under low light and very nice fire under indirect sunlight or the grading lamp. 4 Very Bright Shows good color under low light and sharp crisp color under indirect sunlight or the grading lamp. 5 Brilliant Shows exceptionally bright crisp color under indirect sunlight or the grading lamp, and often shows even brighter in subdued light. CONSISTENCY Sameness in all the relevant characteristics of an Opal, including color, pattern, density of fire and color of the background. OPALESCENCE The milky or pearly appearance of some common Opals. No play of color. OPALITE A plastic Opal simulate. SOLID OPAL Consisting only of Opal, with no other type of stone present and naturally occurring in one piece. ASSEMBLED OPAL Opal that has been glued together, either with another opal or other material. Could be Doublets or Triplets. CRAZED OPAL Opal which shows cracks interwoven into a spider web-like design. Can be seen best when a penlight is shown through the bottom of the Opal. Extremely Fragile! Opal was also treasured in the Middle Ages and was called ophthalmios, or eye stone, due to a widespread belief that it was beneficial to eyesight. Blonde women wore opal necklaces to protect their hair from losing its color. Some thought the opal's effect on sight could render the wearer invisible. They were recommended for thieves! Opal as Muse A beautiful opal called the orphanus was set in the crown of the Holy Roman Emperor. It was described as follows: "as though pure white snow flashed and sparkled with the color of bright ruddy wine, and was overcome by this radiance." This opal was said to guard the regal honor. Opals are also set in the crown jewels of France. Napoleon gave Josephine a beautiful opal with brilliant red flashes called "The burning of Troy," making her his Helen. Shakespeare found in the opal a symbol of shifting inconstancy, likening play of color to play of mind in one of the most apt uses of gemstone symbolism in literature. In Twelfth Night, he writes: "Now the melancholy God protect thee, and the tailor make thy garments of changeable taffeta, for thy mind is opal." In the nineteenth century, opal was considered unlucky due to the plot of a popular Sir Walter Scott novel of the time. The heroine of the novel has her life force caught in the beautiful opal she wears in her hair and she dies when the fire in the opal is extinguished. Queen Victoria loved opals and often gave them as wedding presents. She and her daughters created a fashion for wearing opal. Queen Victoria was one of the first to appreciate opals from an exciting new source: Australia. Ancient opal came from the mines near Cervenica, Hungary, in what is now Eastern Slovakia, where hundreds of men mined the stone. Ancient opal fanciers never had the chance to see the opal of Australia, where the opal of today was born, which far surpasses the beauty of Hungarian opal in fire and brilliance. A Gem of Water, Born in the Desert The story of opal in Australia begins more than 100 million years ago when the deserts of central Australia were a great inland sea, with silica-laden sediment deposited around its shoreline. After the sea receded and disappeared to become the great Artesian basin, weathering 30 million years ago released a lot of the silica into a solution which filled cracks in the rocks, layers in clay, and even some fossils. Some of this silica became precious opal. Opal is one of the few gemstones that is sedimentary in origin. Opal still contains 6 to 10 percent water, a remnant of that ancient sea. Gold panners in Australia found the first few pieces of precious opal in 1863. Mines at White Cliffs began producing in 1890. Only opal with a perfectly aligned grid of silica spheres will show play of color, which is created through diffraction. The size of the spheres determine the wavelengths and therefore the colors seen. The brilliance of the colors are determined by the regularity of the grid. The strength of the colors seen in opal also depend on the background body color and the transparency of the stone. The body color determines the variety of opal and has a large impact on the value. Black opal, opal with a black to dark gray body color, has the most brilliant colors and is the most valuable. Crystal opal, the next most costly type of opal, is transparent with flashes and is highly valued due to the brilliance of its colors and the fact that many layers of color within the stone can also be seen. White and milky opals tend to have more diffused colors due to the light background color. This is the most affordable type of opal. Another more unusual type of opal is boulder opal, which has opal with an ironstone host rock matrix which creates a natural dark background to view its fire. These sometimes occur in "splits" a matched pair of opals created when a piece of boulder opal is split along the opal vein. These are particularly favored for earrings, since they are mirror images of each other. Choosing an Opal Within each opal variety, the brillance of the play of color is the most important value factor. After this consideration, the colors seen and the pattern of the colors will also influence value. Generally, opal with red fire is the most valued because opal that shows red will also show other colors when rolled back and forth: it contains the whole spectrum. The pattern of the play of color also influences value. Generally large flashes and broad patterns are more rare and valuable than small pinfire patterns. Black opal is found only in Australia in Lightning Ridge, the most famous opal deposit in the world since it was discovered in 1903, and in Mintabie, which also produces large quantities of light opal. Another large opal producing area in Australia is Coober Pedy, which produces light opal. The name Coober Pedy is an Aboriginal name meaning "white man in a hole." If you visit Coober Pedy, you will understand how it got its name: many houses - and even a church! - are burrows dug into the ground called dugouts. This type of dwelling is quite practical and cool as temperatures soar in the daytime. Andamooka is known for producing crystal opal and light opal. Boulder opal is produced in several areas in western Queensland. In addition to Australia, a small quantity of precious opal is produced in Brazil. Mexico and the state of Oregon in the United States produce a volcanic opal called fire opal. Fire opal is transparent opal ranging in color from colorless to yellow, orange, and red. Sometimes it also shows play of color in addition to its bright orange body color. Low quality opal was recently discovered in Ethiopia. Opal is cut in Australia, Hong Kong, Mexico, Germany, and other places. Calibrated sizes are widely available in light opal, which is very popular with jewelry manufacturers around the world due to the beauty even of inexpensive pieces. Black opal is cut in free sizes due to its rarity and high value. Boulder opal is often available in the natural shape of the rough. Fire opal can be found in both faceted and cabochon cuts, including many interesting fancy shapes. A green translucent opal that resembles chrysoprase or jade, which is called prase opal, is found in Tanzania. A beautiful blue-green opal is found in Peru in the Andes Mountains. These types of opal do not display play of color. The hardness of opal ranges from 5.5 to 6.5 on the Mohs scale. It should be protected from heat and strong light, which can dry it out, causing cracks. Ultrasoni Opal fire Hardness 5.5 - 6.5 Occurrence Australia, former Czechoslovakia, U.S., Brazil, Mexico and southern Africa. Opal and tourmaline are the birthstones of the month of October. Appearance Iridescent, and opalescent with a wide range of internal colors. Enhancements Opal is rarely impregnated with colorless oil, wax and resins.
Opal Information
Opal information loose sale price & solid Australian doublet Opal Gemstone Information

Did you know that 95% of the world's precious opal comes from Australia?

How Opal Color is Produced
It took the development of the electron microscope to work this out. Precious opal is made up of tiny uniform spheres of transparent hard silica, which fit together in an orderly three dimensional frame, sitting in a "bath" of silica solution. It is the orderliness of the spheres that separates precious opal from common opal.

Light passes through the transparent spheres in a direct line, but when it hits the 'bath' of silica, it is bent and deflected at different angles, thus producing a rainbow effect.

Deflection & Diffraction
Depending on the size of the spheres, varying colors of the spectrum are diffracted. So it is a combination of deflection (bending) and diffraction (breaking up) of light rays that creates the color in opal. If you move the stone, light hits the spheres from different angles and bring about a change in color. The name opal actually means "to see a change in color." The way in which colors change within a particular stone as it is rotated and tilted is called the stone's play of color.

How color is defined
The size of the spheres has a bearing on the color produced. Smaller spheres bring out the blues, from one end of the spectrum. Larger spheres produce the reds from the other end. The more uniform the spheres are placed, the more intense, brilliant and defined the color will be.

How Opal Formed
The consensus among geologists is that almost all Australian gem opal has formed by precipitation of silica from very dilute silica solutions or colloidal suspensions, derived from the deep weathering of feldspathic sedimentary rocks under the action of percolating groundwater.

This deep weathering is a chemical alteration.  The feldspar minerals are altered to kaolinite, releasing silica in an aqueous solution or suspension which may collect in traps or cavities in the rocks.  These cavities may be open fissures, interstices between particles in a conglomerate, holes left by dissolution of shells, bones, wood or inorganic minerals (such as gypsum and calcite), or hollow cores and cracks in ironstone concretions.

The ironstone concretions may be localised along the base of palaeochannels, on the downthrown side of differential compaction faults, or along basal undulations of bedding interfaces.  The silica solution or suspension may have been concentrated by evaporation through the overlying sediments, or concentration may have been effected by the action of clay beds which in many places underlie the opal-bearing horizons, as semi-permeable membranes allowing the water to pass through but retaining the silica.

Increased concentration my have caused the separation of discrete silica particles which aggregated by collision into equidimensional spheres.  When these reached a particular size, they may have undergone undisturbed settling and hardening into an ordered arrangement which formed precious opal.  More commonly, a disordered arrangement of spheres accumulated during settling forming common opal or 'potch'.  Because water is retained in the opal structure, application of heat, or in some cases merely exposure to a dry atmosphere, can cause fractures to develop.

Opaline silica is common throughout the leached profile of the Cretaceous Winton Formation , which consists of feldspathic sandstone, siltstone, and mudstone.  In some places, the matrix of the sandstone is replaced by opaline silica, which may also replace wood and gypsum.

Precious opal, though sometimes found in these modes, is most commonly associated with various forms of concretionary ironstone.



Glossary of Terms
Andamooka Matrix - A more porous opal found in the mining field of Andamooka. When it comes out of the ground it is quite pale but by treating it with a carbon dye process it eventually looks like real black opal. An Andamooka matrix opal can be an affordable alternative to a genuine black opal--but buyers beware--an honest opal dealer will tell you whether you are buying an Andamooka matrix or a genuine black; a disreputable dealer may not.

Amorphous - Shapeless. Not consisting of crystals. Non crystalline. Glass is amorphous. Sugar is crystalline.

Black opal - Opal which is found with a natural black or dark background. This background can range from pitch black to gray giving the stone are darkish appearance when seen from the top. It is the dark background which allows the brilliant colors. Black opal can be any color.

Black Crystal Opal - Is gem colors on a smoky, transparent background, some people regard this as the best type of opal ever found.

Body Color - This is the background color of the opal (not the color of the "flashes"); and is described as being from white, to gray, to black.

Boulder opal - Similar to black opal except that the foreground color is very thin and sits on a brown ironstone base. These stones at times can come with interesting "hills" and "valleys" on the surface. (meaning that the surface is often, but not always, undulating) These stones are preferred by people who are more "progressive" in their jewelry tastes. Often boulder opal has ironstone inclusions in the foreground and all sorts of odd shapes which makes them a designers delight.

Broken Flash - Part of the stone displays a flash of color at a time.

Consistency of Fire - This has to do with the stone's ability to show play of color when viewed from any angle. In other words, the fire is non-directional. The opposite, directional, indicates that fire is visible only when viewed from a certain angle, and are less valuable.

Crystal opal - An opal with a brilliant crystal appearance allowing you to look down into the stone.

Deflection - The bending of rays of light from a straight line.

Diffraction - The Breaking up of a ray of light into either a series of light and dark bands, or into colored bands of the spectrum.

Diffuse - To spread out so as to cover a larger space or surface. To scatter.

Fiery Opal - Predominantly red pay of color sometimes seen as pin fire or stardust flashes.

Floral - A flashy effect like a bunch of flowers, seen frequently in Queensland opal.

Fluorescent - A light produced by the electrical stimulation of a gas or vapor. Fluorescent lights have a similar effect on opal as a bright cloudy day--they do not properly bring out the colors in opal.

Gem/Precious Opal - Is defined by the brilliance of colors in the color bar.   If it is on flawless potch (common opal), it is considered more valuable than on a mixed layer of potch.

Hardness - Opal is 5.5 to 6.5 or harder, similar to Emerald.

Harlequin - Is surely at the top of the list and the most prized of all brilliant opals.   Such rarities portray spangles of rounded, angular to roughly square patches, presenting a harlequin appearance of interchanging colors.

Hydrate - A compound produced when certain substances chemically combine with water.

Hue/Fire Pattern - This describes the geometrical relationships of the colors. Mosaic (a "tiled" look), pinfire (lots of small/tiny spots of color), and flash (broad, sweeping, 3-dimensional color swaths that depend on the angle of viewing) are typical terms.

Hues and Intensity - The predominant and secondary hues are described for the play of color (flashes) present, along with a percentage of the stone "covered" by each color, and each color's intensity from poor to excellent. Other hues (tertiary, etc.) present may be listed if significant.

Jelly Crystal opal - A solid crystal opal that is extremely translucent, to the point of being almost transparent.

Incandescent - Glowing with heat (red or white hot) as in a light bulb which glows white hot, but produces a light that more closely simulates natural sunlight. Sunlight and incandescent lights bring out the natural colors in opal.

Matrix Boulder Opal - A porous sedimentary ironstone having minute cavities filled with precious opal.   When cabochon cut, the dark brown ironstone sparkles with vivid pin flecks of color

Mosaic Opal. The previously described opals are all naturally formed. The mosaic opal is a man-made creation. Slivers of genuine opal are assembled into an irregular tiled pattern with rivers of black potch framing each unique opal tile. Mosaic opals make stunning stones for all types of jewelry and allow the jeweler to include a wide range of colors within an affordable piece.  Now, there is a natural stone that looks very similar to mosaic opal. The natural phenomenon is called the harlequin opal. It is the rarest and most expensive opal.

Opal - Opal comes from the Latin word opalus which means to see a change in color. Chemically, opal is hydrated silica, similar to quartz.  

Specs:
Color: white to gray to black, blue, green, orange
Hardness: 5.5 to 6.5
Specific Gravity: 2.0 to 2.2
Cleavage: none
Fracture: conchoidal, splintery, brittle
Refractive Index: about 1.45
Transparency: transparent to opaque
Chemical Composition: SiO2 . nH2O

Opalescence - A play of color, similar to that of an opal.

Opal fossils - Opalized shells, crustations, sea creatures, snails, animal claws and bones, are some of the many objects that over time can opalize when buried in the unique environment of Australian clay and left undisturbed.

Opalized Wood - Because the creation of opal is a natural process, opals can sometimes be found within substances other than stone.

Opal patterns -  May be classified as follows: pinfire (small pinpoints of color), flashfire (larger irregular pattern of color), broadflash (sheets of color across large portions of the stone), rolling flashfire (sheets of color that "roll" across the stone as it is moved), and harlequin (square/angular blocks of color set closely together; rare).

Opaque - Not allowing light to pass through. The opposite of transparent.

Peacock - Resembling the tail of a peacock. Radiating from one central point and fanning out like the tail.

Picture Opals - Uniquely patterned boulder opals where an object is clearly visible. The specimen has been named "the shrimp." Picture opals are both rare and unique, making them much sought after by opal collectors.

Pin Fire - Seen as pinhead spots in one predominant color such as green.  The most outstanding being a color unit, perhaps red, radiating one way, which changes to another, perhaps green, when turned at various angles.  Some pin fire colors blend into peacock tail design either on part of the stone or from a central nucleus.  Other pin fire colors blend into fern or foliage design.

Play of Color - The way in which colors change as an opal is tilted in different directions.  Play of color in an opal, (its "flashes" or "fire") is caused by light interference patterns created by the tiny spheres that make up opal's structure. These spheres are about 0.001 millimeters in diameter!

Potch (Common Opal) - Common opal with no play of color.   Varies from black to milk white to greenish, yellowish or bluish in color.

Rainbow Opal - The colors are positioned as in a rainbow.

Rolling Flash - Brilliant colors rolling across the face of opal, some having cat's eye rolling effect.

Semi-Black opal - The semi-black opal is found on most fields. It's background color ranges from gray to near black. One of its distinguishing characteristics is an almost smoky appearance. Semi-black opals are from the same family as black opals, but they are not as dark.

Silica - (Silicon Dioxide) A hard, white or colorless substance, that in the form of quartz, enters into the composition of many rocks and is contained in sponges and certain plants. The needle in the mouth of a female mosquito is made of silica. Flint, sand, chalcedony, and opal are examples of silica in different forms.

Specimen - A chunk of opal that is either left in its natural state or given a polish or varnish to show off one aspect or feature of the stone. Opal specimens are sought after by rock collectors, who generally display them on a desk or shelf.

Spectrum - The band of colors formed when a beam of white light passes through a prism or by some other means (e.g. mist or spray, in the case of a rainbow) The full range of spectrum colors are: red, orange, yellow, green, blue, indigo, and violet.

Sphere - A round three dimensional geometric shape whose surface is equally distant at all points from the center point.

Translucent - Letting light through without being transparent.

Transparency - The "see-throughness" of the body is described (from watery to opaque).

Transparent - Easily seen through (glass like)

White opal - A solid opal with an opaque light background.

Yowah Nut -  Brilliantly colored opals nested in a nugget of ironstone.  Though the opal center is sometimes separated from its ironstone nugget, many beautiful and unusual jewelry pieces are made by keeping the nugget and its center-opal together.

 

Variety
 Description
 Image
Agate Opal  Banded variety of Opal.
May also refer to Agate with alternating bands of Opal.  Agate Opal
Amatite  Opal in the form of thick mounds, formed from hot silica-rich springs. See also Geyserite.  
Amber Opal  Opal with a brownish to yellowish background color, resembling Amber.  Agate Opal
Andamooka Opal  Opal from Andamooka, South Australia.  
Banded Opal  Form of Common Opal with color bands. Synonym of Agate Opal.  Agate Opal
Bandfire Opal  Precious Opal with play of colors in wavy bands.  
Black Opal  Precious Opal with a black, dark blue, dark green, dark gray or similar darkly colored background or base color. Black Opal is the most valuable form of Opal.  Unpolished Black Opal
Bone Opal  Opal pseudomorph after a bone.  
Boulder Opal  Precious Opal from Queensland, Australia, found in the cracks of, or as coatings on, ironstone or sandstone boulders.  Polished Boulder Opal
Cachalong Opal  Opaque, highly porous type of Common Opal.  
Cherry Opal  Orange-red to bright red variety of Mexican Fire Opal.  Cherry Opal on matrix
Chloropal  Common Opal similar to Prase Opal, but with a lighter green hue.  
Chrysopal  Common Opal similar to Prase Opal, but with a golden-green color.  
Claro Opal  Transparent Precious Opal from Mexico with an intense red, green, blue, and yellow play of color.  Polished Claro Opal
Common Opal  Any Opal lacking play of color.  amorphous Hyalite Opal with a yellow tinge
Contra Luz Opal  Precious Opal where the play of color is visible only when a light source is behind the stone.  
Coober Pedy Opal  High quality Precious Opal from Coober Pedy, South Australia.  
Crystal Opal  Transparent to translucent Precious Opal where play of color is visible on the surface and in the interior of the stone.  
Dark Opal  Synonym of Black Opal.  Unpolished Black Opal
Diatomite  Opal replacement of microscopic shells of diatoms (type of microscopic organism) clustered together. It is white, opaque, and chalky in texture. Synonym of Tripolite, Fuller's Earth, and Diatomaceous Opal.  
Fire Opal  Fire Opal is incorrectly used to describe Precious Opal, or Opal with play of color. The true definition of Fire Opal is Opal with an orange to red color. If the Fire Opal displays play of color, it is more correctly known as Precious Fire Opal.  Unpolished Mexican Fire Opal
Flame Opal  Precious Opal where the play of color consists of red streaks or bands that flicker like a flame when the stone is rotated.  
Flash Opal  Precious Opal with large schillers that abruptly appear and disappear as the stone is rotated.  Flash Opal
Flashfire Opal  Synonym of Flash Opal (above)  Flash Opal
Fossil Opal  Opal pseudomorph of organic matter such as shell, bone, and trees.  
Gelite  Opal (or Chalcedony) as an accessory mineral that acts as the bonding agent of Sandstone or other cemented rock fragments.  
Geyserite  Opal formed from deposition of hot water springs. Also called Perlite, Fiorite, or Geyser Opal. See also Amatite.  
Gilson Opal  Synthetically produced Opal.  
Girasol  Yellow or orange variety of Precious Opal in which the play of color seems to follow the sun as the stone is rotated.  
Glass Opal  Synonym of Hyalite  amorphous Hyalite Opal with a yellow tinge
Gold Opal  Common Opal with a golden hue.  Faceted Gold Opal
Harlequin Opal  Precious Opal in which the play of color is arranged in a consistent harlequin, diamond-shaped, or rectangular-shaped pattern that is very vivid. Harlequin Opal is one of the rarest and most prized forms of Opal.  
Honey Opal  Transparent to translucent Opal with an orange to orange-brown, honey-colored background. It may or may not display play of color.  Faceted Honey Opal
Hungarian Opal  Any Precious Opal from Europe. However, nowadays this term often refers to any White Opal, regardless of where it was found.  Polished White Opal
Hyacinth Opal  Synonym of Girasol  
Hyalite  Colorless, misty-blue, or sky-blue transparent variety of Common Opal. Usually forms botryoidal masses as well as strange and unusual forms. All Hyalite fluoresces green.  amorphous Hyalite Opal with a yellow tinge
Hydrophane  White, opaque, highly porous Opal, that, when placed in water, allows the water to seep into it. This causes the stone to become transparent and almost invisible while in the water.  
Iridot  Old name given to Opal for a short period of time when Opal had a reputation of causing bad luck.  Polished White Opal
Isopyre  Impure, dark red form of Opal. Isopyre was once thought to be a separate mineral.  
Jasper Opal  Brecciated Jasper in which the cementing material is Opal.  
Jelly Opal  A transparent Precious Opal with a gelatinous appearance and a bluish sheen. Jelly Opal may also refer to a colorless, transparent Common Opal.  Jelly Opal with play of color
Lechosos Opal  Precious Opal with a milky-white background color displaying a strong play of color. May also refer to Opal with a strong green schiller.  Polished Lechosos Opal
Lemon Opal  Common Opal with a lemon-yellow color.  Faceted Lemon Opal
Levin Opal  Precious Opal with long and thin, lightning-like flashes.  
Light Opal  Synonym of White Opal.  Polished White Opal
Lightning Ridge Opal  Opal from Lightning Ridge, New South Wales Australia. Although different forms of Opal are found there, this term usually represents the high quality Black Opal found there.  Unpolished Black Opal
Lithoxyl Opal  Wood Opal where the original structure of the tree is very apparent.  
Liver Opal  Synonym of Menilite (below)  
Menilite  Opaque, grayish-brown form of Common Opal. Also known as Liver Opal.  
Mexican Fire Opal  Form of transparent Opal from Mexico, usually with an orange or red colors, highly desired as a gem. Although scientifically considered a Common Opal, it is rather rare and much sought after. If it exhibits a play of color, it is known as Precious Fire Opal.  Unpolished Mexican Fire Opal
Milk Opal  Opal with a milky-white color. Controversy exists whether the name Milky Opal is coined for a milky white Common Opal or a milky white Precious Opal.  Unpolished Milk Opal
Moss Opal  Common Opal containing inclusions resembling moss.  
Mother of Opal  Precious Opal with bright color specks filling the pores of sandstone or ironstone.  
Mother of Pearl Opal  Banded Opal used as cameos.  
Mountain Opal  Opal from igneous environments. Also called Volcanic Opal.  
Neslite  Common Opal similar to Menilite, but darker gray in color. It was once a popular material for sword handles.  
Nevada Opal  Opal from the Virgin Valley (Humboldt Co.), Nevada.  Flash Opal
Onyx Opal  Common Opal resembling banded Onyx.  
Opal Matrix  Thin layer of Precious Opal on host rock. Small rock fragments are used in jewelry.  
Opaline  Opaline is synonymous with Opal Matrix (above), but was also an old term used to describe Opal from Australia.  
Opalite  Opalite has many connotations. It may refer to:
Opalized Bone  Synonym of Bone Opal  
Opalized Fossil  Synonym of Fossil Opal  
Opalized Shell  Synonym of Shell Opal  Shell Opal (Opal replacement of shell)
Opalized Wood  Synonym of Wood Opal  
Painted Boulder  Sandstone boulders with a coating of Precious Opal. When used in jewelry, this term is synonymous with Opal Matrix.  Polished Boulder Opal
Pearl Opal  Synonym of Tabasheer  
Pineapple Opal  Opal pseudomorph after Ikaite that resembles a pineapple. It is found only in White Cliffs (New South Wales), Australia. The pseudomorphed mineral was originally thought to be Glauberite, but studies now prove it to be Ikaite.  
Pinfire Opal  Precious Opal with very small, pinhead-size color flashes.  Polished Pinfire Opal
Pinpoint Opal  Australian synonym of Pinfire Opal  Polished Pinfire Opal
Pipe Opal  Opal formed as a filling of long, cylindrical cavities in rock. Pipe Opals range in size from several inches to many feet.  
Pitch Opal  Yellow to brown Common Opal with a pitchy luster.  
Potch  Australian term for Common Opal.  amorphous Hyalite Opal with a yellow tinge
Prase Opal  Green to dark green form of Common Opal.  
Precious Fire Opal  Fire Opal displaying play of color.  Polished Precious Fire Opal
Precious Opal  Any Opal exhibiting a play of color.  Polished White Opal
Prime d'Opal  Synonym of Mother of Opal  
Pyrophane  Precious Opal in which the play of color wanders about and reappears at random. This term is sometimes incorrectly used to describe Girasol.  
Queensland Opal  Synonym of Boulder Opal  
Quinzite Opal  Rose to pink colored Opal. It is usually without play of color, but a few examples displaying play of color are known. Quinzite Opal is synonymous with Quinzite, Quincite, Quincite Opal, and Rose Opal.  Quinzite Opal
Radiolite Opal  Common Opal of a smoky-brown color caused by inclusions of the exoskeletons of a unicellular marine organism known as radiolaria. May also be called Radio Opal.  
Rainbow Opal  Precious Opal where the play of color is seen in curved bands, somewhat resembling a rainbow.  
Red Flash Opal  Precious Opal with red color flashes that swiftly appear and disappear as the stone is rotated.  
Resin Opal  Common Opal with a yellow-brown color and resinous luster.  
Rumanite  Opal from Romania.  
Seam Opal  Opal found in the seams or large cracks of rock. May also specifically refer to masses of white Common Opal containing bands of precious White Opal.  
Semiopal  Term sometimes used to describe any type of Common Opal, but many times alludes to particular forms of Common Opal, such as Wax Opal, Prase Opal, etc. Semiopal is also written as Semi-opal, and is synonymous with Half-opal.  
Shell Opal  Opal pseudomorph after a shell.  Shell Opal (Opal replacement of shell)
Slocum Stone  A synthetically grown Opal. Also called Slocum Opal.  
Sun Opal  Name that describes several types of Opal. May refer to Fire Opal, Mexican Fire Opal, Honey Opal, or Amber Opal.  Faceted Gold Opal
Tabasheer  Opal occurring as an organic byproduct. It forms by the hardening of a secretion issued from certain bamboo, forming a porous, rounded mass of Opal.  
Virgin Valley Opal  Opal from the Virgin Valley (Humboldt Co.), Nevada.  Flash Opal
Wash Opal  Waterworn Opal pebbles from alluvial deposits.  
Water Opal  Synonym of Jelly Opal  Water Opal with a billowy bluish sheen
Wax Opal  Yellow to brown Common Opal with a waxy luster.  
White Cliffs Opal  Opal from the White Cliffs, New South Wales, Australia  
White Opal  Precious Opal with a light body color (white, yellow, cream, etc.). Differentiated from Black Opal which has a dark background color.  Polished White Opal
Wood Opal  Any Opal that formed a pseudomorph after wood from a tree, and retains the original shape and appearance of the wood. Wood Opal may refer to both Common Opal and Precious Opal, but the term usually refers to large pieces of Common Opal.  
Yowah Nut  Small, rounded form of Boulder Opal from Yowah (Queensland), Australia in a nodules embedded in ironstone. Closely related to Boulder Opal, it occurs most often as walnut-sized ironstone nodules containing pockets, veinings, or sprinklings of vivid Precious Opal.