Glossary of Opal terms loose sale price & Opal Gemstone Information
Alluvial: are sediment deposits (silt, sand, and gravel) resulting from operation of modern streams.
Amorphous: is the term applied to rock and minerals having no definite crystalline structure.
Black Opal: is a rare variety of precious opal, mainly found at Lightning Ridge, characterised by a spectacular play of spectral colours in a dark body (black, blue, brown or grey); its great depth of colour and large pattern make it particularly prized and valuable.
Body Colour: is the background tone of opal, also called base colour.
Boulders: are large rounded stones, the coarsest component of gravel.
Boulder Opal: is distinguished by the presence of host brown rock, usually ironstone, on the back of the opal; and is found almost exclusively in Queensland.
Cabochon: is a circular or oval shape with curved top and flat base (simple cabochon) or with upper and lower portions curved (double cabochon).
Calibrated: is opal cut to standard size and shape, for example, 7mm x 5mm oval.
Capping: refers to the forming, or occurring as a cap or cover on top of a rock. back to top
Carat: is the measure of weight used for gemstones (0.2 gram).
Claystone: are rocks in which much clay is present or which are largely composed of clay.
Colour: is a small piece of precious opal.
Colour Bar: is a layer of opal differing from colour layers above and below.
Colour Separation Bar: is often confused with cracks by the inexperienced - this fine line may occur where different colour formations meet.
Cotton: is a thread-like impurity in opal.
Crack: refers to a fracture in opal, which greatly reduces the value.
Craze: is a fracture in opal, which greatly reduces the value.
Cretaceous: is a time period approximately 136 to 64 million years ago in the Mesozoic era.
Crystal: refers to transparent or translucent opal where the colours are sharp and visible below the surface.
Diffraction: is the breaking up of white light causing spectral colours.
Doublet: is composed of a layer of precious opal attached to a dark background; simulates black opal. back to top
Fire: refers to the diffracted colour play of an opal.
Fire Opal: is a term used for opal where the base colour (as distinct from the diffracted colour) has red and orange tone. This material usually comes from Mexico.
Flaw: is a mark or impurity that devalues the stone.
Freeshape: is an opal cut into an irregular shape.
Freesize: refers to an opal not cut into a standard calibrated size.
Gem Quality: is the finest grade of opal.
Gemstones: are any precious or semi-precious stones, whether as mined, or cut and polished; for example, diamond, opal, etc.
Harlequin: is the rarest of all patterns of black opal showing a quilt of small blocks of colour in every shade of the rainbow. It is often said to be the rarest gemstone on earth.
Impurity: is a foreign particle in the stone such as sand spot or gypsum in opal. Jelly opal: is clear, almost colourless opal. back to top
Light opal: refers to opal of a white or light blue appearance with a play of various colours.
Matrix: refers to
1. Porous opal from Andamooka often treated to simulate black opal; 2. Boulder 'matrix' opal showing flecks of flashing colour scattered throughout the brown ironstone; and 3. A term used by miners at Lightning Ridge to describe a porous white impurity in black opal.
Milky: is opal with creamy body colour that dominates the diffracted colour.
Nobby: refers to a type of rough opal occurring in Lightning Ridge - a round nodule.
Nodule: is a small more or less rounded body.
Opal: is a precipitated non-crystalline variety of silica, precious varieties of which characteristically display a marked variety of changing colours and are used as gemstones, especially when cut and polished.
Potch: is common non-precious opal exhibiting no play of colour.
Red On Black: refers to black opal displaying predominantly red colour, and is extremely valuable. back to top
Rough opal: refers to uncut and unpolished opal.
Sandstone: is the sedimentary rock composed of cemented or compacted sand-sized particles, predominantly composed of quartz.
Seam opal: is opal occurring in horizontal seams. It is the most common type of formation, and can be found in all opal fields.
Semi-black: is opal with a dark background but not dark enough to be termed 'black opal'. Usually mined at Mintabie.
Solid: refers to opal that has not been backed, capped or treated by man.
Straw pattern: is a rare pattern-the colours in the opal look like straws.
Tertiary: is a period of the Cainozoic era approximately 64 to 1.5 million years ago.
Triplet opal (Triplex): is a thin slice of light opal attached to a black background with a protective crystal dome glued on top. back to top
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Swiss Gem Shop loose Gemstone at Sale price and Gemstone Information