Valuable opals have been reported from several states in the western part of USA. They are found in tiny quantities in volcanic rocks in the remote area of western states, from California all through to Washington, but the most significant occurrence is at Rainbow Ridge in Virgin Valley, Nevada.
The region of Rainbow Ridge comprises the leading and the most recognized deposit in USA. The accurate date of the discovery of opal has been an issue for discussion amongst the old locals of the area. It is usually accepted in either 1905, or in the middle of 1906, and some people say during 1908.
A large number of Virgin Valley opal is prone to breaking and fracturing, so that most of the opal discovered is trade and remains as an uncut variety.
The area is known for its unusual fire play in wood, limp casts and bark replacement. They are Black in color and are found in Humboldt County, Nevada. Their black matrix contains stunning flashes of red, blue, purple, and green. Virgin Valley opal is believed to be about 14 -15 million years old. The opal has a tendency to craze when dried too fast as it contains up to 14% water. Some cutters store the opal in mineral oil for a year and change it every 3 months before cutting.
This Opal has colors that are formed in stripes similar to bacon. Opals can be slightly translucent and can have a mixture white, pink, red or yellow stripes.
This opal was formed in volcanic rocks around 4 million years ago by geysers and is found in Idaho USA. When a geyser erupted, the liquid contains tiny spheres of silica which would form thin layers of precious opal.
The opal colour bar is very thin but ideal to make opal triplets. This opal can be stabilized as same treatment to Ethiopian opals.
The opals are found in rhyolite geodes (thunder eggs) in a perlite that has altered to pastel colored clay. The geodes that contain gem-quality opal are only about 10% of the total geodes mined and only about 1% of the geodes contain gem-quality opal with play of color. The remaining geodes contain agate, quartz crystals, or common opal. The geodes vary greatly in size, from a few centimeters in diameter to over a meter