Opal-A

specimen number:
2133427

location:
Kernville, Kern Co., California, USA

description:
Colorless botryoidal crystalline opal-A on matrix. In good condition.

Opal-A is an amorphous, hydrous silica mineraloid consisting of highly disordered, closely packed networks of sub-microscopic silica spheres that contain up to ten percent water by weight. Hyalite is a specific colorless, water-clear variety of Opal-A that typically forms botryoidal, globular, or crust-like encrustations within volcanic cavities, pegmatites, or late-stage hydrothermal environments. Geologically, hyalite precipitates directly from silica-saturated fluids at relatively low temperatures, often forming thin coatings over host rocks or associated minerals. Unlike precious opal, the random structural arrangement of its silica spheres lacks the regular, periodic spacing necessary to cause the diffraction of light, meaning hyalite exhibits no play-of-color under normal light conditions. However, many hyalite specimens contain trace amounts of uranyl ions incorporated during crystallization, which causes the material to display a strong, vivid green fluorescence when exposed to short-wave or long-wave ultraviolet light.

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