Hyalite Opal, Electric Opal Knob, Zacatecas, Mexico

The Electric Opal Claim in Zacatecas, Mexico produced specimens of hyalite opal that were notable for having brilliant green fluorescence under UV, and bright yellow/green daylight fluorescence when viewed under shaded daylight outdoors. The opal was found in welded volcanic tuff that formed a distinct layer in a knob on top of a mesa composed of material from volcanic outflows. The Electric Opal Claim produced many high quality specimens of hyalite between 2013 and 2018, but due to the limited size of the opal-bearing layer, has been subsequently mined out. The specimen featured here is typical of the finds extracted from Electric Opal Knob, and consists of colorless to greenish, transparent, bubbly hyalite opal covering a brown/grey colored matrix. The hyalite opal shows a bright green fluorescent response under both SW and LW UV, and also exhibits yellow/green daylight fluorescence. This specimen is moderately radioactive and its emission was measured at 200-400 cpm, evidence of its uranyl-activated mechanism of fluorescence.

As shown above, the hyalite shows bright green fluorescence under short wave UV (254 nm).

This hyalite is also fluorescent under long wave UV (365 nm), but the response is not as bright as seen under short wave UV.

Same specimen shown under visible light (lighting provided by an LED lamp).

When taken outside on a cloudy day, the hyalite shows a yellow/green daylight fluorescent response. The daylight fluorescence is excited by the long wave UV (365 nm) and violet/blue (405 nm) components of sunlight.

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