GLOSSARY OF TECHNICAL TERMS

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.

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.

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.

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.

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.