Wednesday, July 8, 2009

Precious opal from Louisiana is a sandstone

The reported precious opal from Louisiana is a sandstone/quartzite with precious opal cement and matrix. It has blue or purple play of color. The material could be cut into cabochons for jewelry and other items of interest. To date, most of the material has been cut into large (over 2-inches in diameter) gemstone spheres.

Nevada is known for precious opal from Virgin Valley. The first discovery of precious opal in the Virgin Valley area was in 1905 or 1906. Since then a significant quantity of the highly prized opal has been recovered. Virgin Valley opal is comparable to any in the world for its vivid play of color and in terms of the size of material available. Individual pieces weighing over 3 kilograms have been recovered from the Virgin Valley deposits. In 1993, miners found a 100-kilogram opalized log containing precious opal. The material varies in body color from deep pure black to brown to yellowish-white to white to colorless. The play of color includes all colors common to precious opal, red, blue, green, yellow, orange, etc. The opal occurs primarily as replacement of wood, or sometimes, as replacement of conifer cones. Some opal does occur as nodules filling void spaces in clay. The wood replacement is so complete, that generally the wood grain and banding are no longer visible. The exception to this would be that often the exterior wood texture is still present as a brown or black rind.
The uses of the opal can be restricted because of crazing. Crazing is the breakdown or deterioration of opal by the development of very fine cracks all over the surface that extend until they intersect. In the worst cases, the surface of the opal deteriorates into a crumbling sand-like material. Because of the crazing the opal is not well suited for use in jewelry, but displayed in water, glycerine, mineral oil or other liquids makes remarkably beautiful mineral specimens. The mineral collections of most of the better museums contain very fine pieces of Virgin Valley opal. Many museum pieces are crazed from exposure to the air.

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