The opal here can fall apart. Museums display it under liquid for exactly that reason.
The problem is called crazing, and the US Geological Survey describes it plainly: the breakdown of opal through the development of very fine cracks all over the surface, spreading until they intersect. In the worst cases the surface deteriorates into a crumbling, sand-like material. It is why this opal is not well suited to jewellery, and why displaying it in water, glycerine or mineral oil makes it into a remarkably beautiful specimen instead. Most of the better museum collections hold fine Virgin Valley opal, and many of those pieces have crazed from exposure to air.
Everything else about the deposit is superlative. The district lies in Humboldt County, Nevada, near the Oregon border inside the Sheldon Game Refuge, and the survey rates its opal as comparable to any in the world for vivid play of colour and for the size of material available. Individual pieces over three kilograms have come out. In 1993 miners found an opalised log weighing a hundred kilograms. The first discovery of precious opal here was in 1905 or 1906.
Much of it is wood. The opal occurs as a replacement of wood, and sometimes of conifer cones, and the replacement is usually so complete that the grain and banding have gone entirely, though the exterior wood texture often survives as a brown or black rind.
The setting is volcanic ash. Numerous discontinuous layers of opal run through a gently dipping series of vitric tuff and ash at least 300 feet thick, capped by dark vesicular basalt on the east and a thin sheet of terrace gravels along the west side of the valley.
You can dig it yourself. Through the summer at least two fee mines in Virgin Valley are open to individuals, one run by the Hodson family and one by the Wilson family, where you pay a daily fee, dig, and keep everything you find. Other mines in the area are commercial producers.
One thing to know before you fill a bucket. The survey came here in 1951 to look for uranium, examining and radiometrically testing nineteen claims then owned by Jack and Toni Crane, and found carnotite, a uranium mineral, as fracture coatings and fine layers within the opal.
The Rainbow Ridge, Royal Peacock and Bonanza mines here are public fee-digs, you pay to dig your own black opal from the old lake beds.
Black opal
Other places on the map within a couple of hours.
