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It has hardness of 5.5-6.5 Mohs. It is formed of hydrated silica and its specific gravity is 1.98 - 2.25.
Nature's entire splendor seems to be reflected in the manifold opulence of fine Opals: fire and lightings, all the colours of the rainbow and the soft shine of far seas. Australia is the cla ssical country of origin. Almost ninety-five per cent of all fine opals come from the dry and remote outback deserts.
The following varieties of gemstones come under it -
White opal: Opaque, porcelain-like white material; colors resemble flashes or speckles
Black opal: Flashes and speckles appear against black background
Water opal: A transparent, colorless opal is the background for brilliant flashes of color
Fire opal: Reddish or orange opal
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