Hyalite Opal, Electric Opal Knob, Zacatecas, Mexico

Contributed by: Chris Clemens
minID: 1NT-W9R
Date: Apr 18th, 2026
Locality: Zacatecas, Mexico (See on Mindat)
Size: 47 x 30 x 69 mm
Weight: 80 g

Description:
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.

Originally posted by Chris Clemens on Nature's Rainbows in 2020.

Fluorescence under shortwave UV light.

Fluorescence under longwave UV light.

Normal light.

Daylight fluorescence. 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.

Summary of luminescence responses:

Opal-AN (Mindat) (RRUFF)

• Fluorescence under Longwave (365nm LED) UV light: Green
• Fluorescence under Shortwave (254nm Lamp/Mercury) UV light: Green
• Fluorescence under Daylight: Green