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Opal Opal is a precious stone composed of silica similar to quartz with a varying amount of water trapped within the mineral structure. 90% of all opal found is valueless "potch" (opal without colour, usually white or greyish white) but the remaining 10% is the brilliantly coloured and highly prized gem. Precious opal often contains 6-10% water. Romans considered opal more precious than all other gemstones, believing that it held the colours, and therefore the combined beauty of all precious gems.
Opal is classed by colour, brilliance, quality, clarity, variety and pattern of the colourful spheres. The main price determining factors are (1) "base colour" ( a black opal has a dark base and is more valuable than crystal opal which is almost transparent, and it in turn is more valuable than the white or milky opal); (2) "dominant colour"; Red fire opal is more valuable than green opal, and in the same way green is more valuable than opal with just blue colour. (3) "colour pattern" refers to the placement of the crystals for example harlequin opal which has colour in defined patches, is more valuable than pinfire opal where the colour is in small specks. Opals that are cut in a solid piece are known as solids. These are the most valuable. A doublet - two pieces - has precious opal laminated to another stone (usually potch or glass or ironstone). A triplet has three parts. A piece of precious opal in the centre, a clear top and a darkened base (usually potch or glass) to highlight the colour.
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