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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.
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.
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.
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.
Opal that has formed naturally on ironstone or "boulder" which is cut to form that host rock.
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.
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.
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.
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 Gemstone Information
Australian black Opal Gemstone Information & Solid Australian Opals loose sale price