gems, opal

Hopkins presents an introductory account of opal, combining historical lore with explanations of its composition, optical effects, principal varieties, treatments, assembled stones and valuation. Traditional beliefs and anecdotes illustrate the fascination inspired by its changing colours, while Australian occurrences provide the main geographical setting. Opal is described as non-crystalline, hydrated silica, formed from silica-bearing material deposited in cracks and cavities; replacement of organic remains can produce opalised fossils. The discussion connects play of colour with the ordered arrangement of minute silica spheres, whose regular structure diffracts visible light. Sphere size influences the colours displayed, while irregular size or arrangement prevents the characteristic effect and produces common opal, termed potch in Australia. Precious opal is distinguished by its play of colour, with further descriptions of black, light, crystal, boulder and matrix material. Black opal is associated particularly with Lightning Ridge and Mintabie, light opal with Coober Pedy, and boulder opal with Queensland ironstone containing seams of precious opal. Treatment methods include carbon darkening of porous matrix opal through sugar and acid processing or smoking, as well as plastic impregnation of lower-quality material. Carbon deposits in cracks are identified as evidence useful in separating treated matrix material from natural black opal. Doublets combine a thin opal layer with a dark backing; triplets add a transparent quartz cap that protects the opal and magnifies its colour effect. The article also mentions Gilson synthetic opal and its characteristic snakeskin or lizard-skin appearance under magnification. Valuation is related to the form of the material, the strength and range of play of colour, and contrast between adjacent colours and against the body colour. Red-bearing displays are especially desirable, but redness alone is considered insufficient without an attractive overall pattern and contrast. Transparent crystal opal with strong colour is highlighted among lighter varieties. The closing discussion stresses fragility and susceptibility to cracking, attributes deterioration to water loss, and advises avoiding heat and purchasing cracked stones.