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One of the two principal varieties of opal (as contrasted with common opal) which exhibits iridescence due to a play of spectral colours when light falls upon its surface. It includes a regular structure of minute, closely set internal spheres of similar size but there may be variations of the sphere size between different stones which regulate the wave lengths of the reflected light, causing varicoloured interference of light. When heated, it loses water, turns dull, and loses iridescence. Most examples are somewhat transparent with backgrounds that are milky or varicoloured.
The colour play shows in different patterns which establish various varieties, including principally the black opal, fire opal, white opal, and water opal, and also flame opal, flash opal, harlequin opal, pinpoint opal, and hyalite. The class is sometimes called 'noble opal'.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson