Hyalite Opal

Hyalite is a colorless, transparent variety of common opal that forms as glassy, globular (botryoidal) crusts resembling drops of clear glass or water. Its name comes from the Greek hyalos, meaning glass. It is also historically called Müller's glass.

Its most striking feature is brilliant green fluorescence under ultraviolet light, produced by trace amounts of uranium (uranyl ions). Some hyalite even has a faint daylight green tint from the same uranium content.

Hyalite is prized more as a fluorescent mineral specimen than as a cut gem, though clear pieces are occasionally faceted.

Hyalite forms by deposition of hydrated silica from low-temperature aqueous solutions or silica-rich vapors onto the surfaces of host rocks, typically volcanic rocks such as rhyolite, basalt, and granite, as well as in cavities and fractures.

Where groundwater carries trace uranium, the uranyl ion is incorporated into the silica, giving the opal its characteristic green UV fluorescence. The botryoidal, droplet-like habit reflects gel-like silica accreting on rock surfaces.

Famous localities include Zacatecas (Mexico), Erongo (Namibia), Hungary, the Czech Republic, and various U.S. volcanic regions.

Look for clear, colorless to faintly greenish glassy blobs or crusts with a botryoidal surface, vitreous luster, and Mohs hardness around 5.5-6.5. The streak is white. The definitive test is a UV lamp (especially shortwave/longwave): hyalite glows vivid green.

Look-alikes: Clear quartz/rock crystal is harder (7) and crystalline with faces, not droplet crusts, and does not fluoresce green. Glass lacks the botryoidal natural habit and the green UV glow. Other opals usually have body color or play-of-color, which hyalite lacks.

The combination of glassy droplet form plus green fluorescence is diagnostic.

Hyalite is mainly a collector's specimen, especially prized by fluorescent-mineral enthusiasts for its dramatic green glow under UV. Display pieces from Erongo, Namibia and Zacatecas, Mexico are particularly sought after.

Clear material is occasionally faceted into novelty gems, but its softness and tendency to craze limit jewelry use.

Because its fluorescence comes from uranium, hyalite is mildly radioactive; handling is generally safe, but specimens should not be ingested or kept in living spaces in large quantities. It has no major industrial use.