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The Virgin Valley opal fields of Humboldt County in northern Nevada produce a wide variety of precious black, crystal, white, fire, and lemon opal.
The black fire opal is the official gemstone of Nevada. Most of the precious opal is partial wood replacement.
The precious opal is hosted and found within a subsurface horizon or zone of bentonite in place which is considered a “lode” deposit.
Opals which have weathered out of the in-place deposits are alluvial and considered placer deposits. Miocene-age opalised teeth, bones, fish, and a snake head have been found. Some of the opal has high water content and may desiccate and crack when dried.
The largest producing mines of Virgin Valley have been the famous Rainbow Ridge, Royal Peacock, Bonanza, Opal Queen, and WRT Stonetree/Black Beauty mines.
The largest unpolished black opal in the Smithsonian Institution, known as the “Roebling opal”, came out of the tunneled portion of the Rainbow Ridge Mine in 1917, and weighs 2,585 carats (517.0 g; 18.24 oz).
The largest polished black opal in the Smithsonian Institution comes from the Royal Peacock opal mine in the Virgin Valley, weighing 160 carats (32 g; 1.1 oz), known as the “Black Peacock”.