Opal Body Tone
Opal Brightness
Opal Shape
Opal Transparency
Brilliant lively on the crystal opals are most
highly prized.
Opal Pattern
Interesting patterns of the diffracted colours are rare and have an enormous impact on the value. Distinct patterns such as rolling flash, straw pattern, harlequin, Chinese writing, ribbon are generally highly sought after.
Doublet and Triplet
Precious Opal usually cut into oval, pear or round shapes. Occasionally other shapes are cut including marquise, rectangle and heart. Non- standard shapes include freeform and baroque.
Treatment
Treated Andamooka matrix opal is subjected to solutions of sugar and sulphuric acid to darken the body tone and enhance the
colour. Composite opals, such as doublets and triplets contain slices of opal cemented to other materials. None treatment stone means natural and solid stone.
Black Opal
Black Opal is a generic term given to any opal which has a dark body colour when viewed from above. “Play of Colour” within a dark or black body colour will enhance the brilliance of the colours. Some have a transparent crystal colour bar overlaying dark opal potch, giving the otherwise light opal a dark appearance. Even expensive black opals may have only a thin colour bar on black potch.
Boulder Opal
Boulder Opal is a variety of precious opal that forms naturally within the host rock, often just a think vein of precious opal is present.
Boulder Opal
Fine grade boulder opals have the top surface covered with precious opal. Slight marks and protrusions of ironstone in the face of the stone are fairly typical, and can affect value. Boulder opal is usually cut to the contours of the opal vein creating a baroque wavy surface and they are often irregular in shape.
Boulder Matrix
Veins or patches of precious opal intimately mixed with the host rock are referred to as boulder matrix opal. This form is found mainly in Queensland. Matrix Opal increases in value with brilliance and as the quantity of precious opal becomes a higher proportion of the face of the stone.
Boulder Splits
These are formed where a single piece of rough is split along the opal vein to form a matching pair.
Opal Nuts
These are commonly referred to as “Yowah Nuts” although they are mined at both Yowah and Koroit in southwestern Queensland. They derive their name from their nut-like appearance. The opal occurs in kernels inside an ironstone concretion,
White Opal
Light Opal is commonly referred to as “white”, although this term should be used only where the body colour is milky and white. Most of white opals are found in South Australia. Brilliant light opal can be extremely valuable.
Doublet and Triplet Opal
A Doublet Opal is a layer of precious opal bonded to Black Backing, simulating black opal. Doublets are usually more expensive than triplets because more opal is used.
A triplet opal is a thin veneer of precious opal bonded to a black backing with a protective crystal dome glued to the top.