Opal Buying Guide - Australian Gemstones, Antiques and Appraisals Magazine

Stop Press! Stop Press! Stop Press! The famous Virgin Rainbow Black Opal is for sale here! Opal Types, Varieties, Trade Names and Values The range of colours of an opal are determined by the size of the silica spheres, which bends and splits light as it passes through the opal, causing spectral colours at different angles. The brighter and more distinctive the play-of-colour (shifting of spectral colours), the better the quality of opal. Not all opals show a play-of-colour.
Opals occasionally crack or craze when cut and set, and with sudden temperature changes. Heating can cause opals to turn white or brownish and can cause the play-of-colour to disappear.	Opals from Lightning Ridge, Australia
Common Opal (Potch): An opal with no play-of-colour and a translucent to opaque transparency. Generally of little value. Precious Opal: The most popular opal which has a play-of-colour. Light Opal includes White Opal, Grey Opal, Crystal and Jelly Opal: Opal with a play-of-colour and a light body colour. White Opal is the most common type of Light Opal. It typically has an off white background colour and can be translucent to opaque. Milky white stones with little play-of-colour are used in budget jewelry. White Opals with a brilliant play-of-colour can retail for a few hundred dollars per carat. Crystal Opals have high transparency, a near colourless body colour and a distinct play-of-colour.
A solid light crystal opal from White Cliffs, NSW, Australia	The Crystal Opal is the most valued of the Light Opals - at 2000 prices, they can cost USD$2500 per carat. A translucent opal with a play of colour is called a semi-crystal opal.
Jelly Opal or Water Opal is near translucent to translucent and has either no or little play of colour. These can be purchased for as little as USD$5 per carat. Better qualities sell for USD$100 - $200 per carat. Black Opal includes Dark Opal, Semi-Black Opal and Black Crystal Opal:
In strict terms, Black Opals have a play-of-colour against a black or very dark background. Black Opal can be used as a generic term referring to an opal with a distinctive play-of-colour against a dark background.	'The Red Heart of Australia' A solid black opal from Lightning Ridge
Semi-Black Opals: Opals with a mid-gray to dark background are classified as Semi-Black Opals. Black Crystal Opal: A dark, translucent or semi-translucent gemstone with a play-of-colour. The classifications between opals vary between dealers. Comparison stones are needed to understand the various depths of body colors and even so it can be difficult to distinguish between some black and semi-black opals. When black opals were first discovered in Australia in 1903, they were not readily accepted, many people assuming they were treated opal or white opal glued to black onyx. Some jewellers considered them to be valueless. Top grade black opal can sell for USD$15,000 per carat. Some exceptional stones have been sold for over USD$20,000 per carat. Boulder Opal: Opal that is still attached to rock, usually ironstone, in which it was found. This opal is found in boulders whose cracks and spaces have been infiltrated with silica. Boulder opal often sells for several thousand dollars per piece, but attractive gemstones can be purchased for a few hundred dollars. For people who cannot afford a colorful black opal, boulder opal can be a good alternative. It costs less because the stone is not solid opal - it is a layer or opal naturally adhering to a rock base. Matrix Opal or Opal-In-Matrix: Stones with lines or spots of opal randomly scattered (mixed) throughout the matrix (host rock in which a mineral, fossil or pebble is found). The most common type of matrix opal found on the market is a porous opal from Andamooka, Australia which is often dyed black to simulate black opal. Yowah Opal, another type of matrix opal is completely natural and is mined in Yowah, South NSW. The patches and lines in its ironstone mtarix often forms distinctive patterns. Matrix opals sell for much less than boulder opals. Fire Opals: A translucent to semi-translucent opal with red, orange or brownish body colour with or without a play-of-colour. Mexico is the principle source of fire opal. Even if it has no play-of-colour, the red and orange opal from Mexico is named fire opal. The most valuable fire opal is reddish, transparent and has play-of-colour. This quality can retail for as much as USD$300 per carat. Translucent gemstones that are yellowish or brownish can sell for around USD$5 per carat. Mounting, Assembled Opals Opal is sometimes impregnated with oil, plastic or wax to improve the play-of-colour and to prevent or disguise cracking. The plastic method is stable, but oil and wax are not stable. There are various techniques for creating the appearance of black opal, including smoke impregnation, backing with black or coloured paint and treatment with dye, silver nitrate or sugar carbonized by acid. Avoid repolishing or applying solvents to treated opals. Composite Natural Opal Opals buyers seeking stones with the appearance of black opal but without the expensive price tag might consider an opal doublet or triplet.
Opal Doublet	An Opal Doublet is a thin slice of opal cemented usually with black glue to another material such as potch opal, chalcedony or glass.
An Opal Triplet is a doublet (of precious light opal) which has a protective crystal dome of colourless quartz or glass glued to the top. Opal doublets are normally more expensive than triplets because more opal is used.
Do not confuse these assembled stones with boulder opals which have a naturally attached rock. One can often detect the man-made stones by looking at the sides - a doublet typically has a straight separation line and a boulder opal has a crooked line.	Opal Triplet
Fake Opals: There are fake opals on the market. Hong Kong is a major producer of imitation opals and synthetic opals are grown in Japan and Russia. Fake opals include Slocum Stones and Opalite. See Factors Determining Opal Values See Opal Care