Precious opal, as compared to common opal, offers the rainbow iridescence that has been so highly prized since Roman times. Australia boasts the largest opals: a 26,350-carat gem-quality white opal found in 1989, and a 1,982.5-carat gem-quality uncut black opal unearthed in 1986. (One carat equals one-fifth of a gram.)

Class: sometimes considered precious
Origin of Name: probably derives from the Sanskrit word upala (precious stone)
Color: pale (white opal); clear (water opal); black, grey, or brown (black opal); yellow, orange, red (fire opal)
Chemical Composition: silica with up to 10 percent water (in precious opal)
Crystal System: non-crystalline or only poorly crystalline
Hardness: 5.5-6.5
Specific Gravity: 1.98-2.20
Geographic Origins: Australia (white and black opal), Mexico (fire and water opal), Brazil, Guatemala, Honduras, Nevada, Oregon, Idaho

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