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.
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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.
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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.
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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.
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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.
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