Opal-A

specimen number:
2133334

location:
Kernville, Kern Co., California, USA

description:
Glassy colorless bubbly Opal-A on matrix. Neat locality example for the species.

Opal-A is an amorphous, hydrous silica mineraloid that represents a highly disordered, non-crystalline form of silica containing a significant percentage of water trapped within its structure. Geologically, it frequently precipitates from superheated, silica-rich hydrothermal fluids or volcanic gases in close proximity to volcanic vents, basaltic cavities, and pegmatite veins, forming smooth, glass-like encrustations and globular clusters. The specific variety historically referred to as hyalite, or water opal, is a chemically pure, water-clear subtype of opal-A that lacks the internal ordered spheres of precious opal, meaning it displays no play-of-color but instead exhibits a striking, glass-like transparency. This material has long been a favorite among mineral collectors due to its unique appearance, and certain specific localities, such as those in Zacatecas, Mexico, have gained international renown because their hyalite specimens contain trace amounts of uranyl ions that cause the otherwise colorless material to fluoresce an intense, brilliant neon green under ultraviolet light.

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