Hyalite Opal - Thomas Range, Utah

Contributed by: Chris Clemens
Date: Apr 29th, 2026
Locality: Thomas Range, Juab County, Utah, USA (See on Mindat)
Size: 63 x 29 x 68 mm
Weight: 77 g

Description:
The Thomas Range, located near the town of Delta in the West Desert area of Utah, is a small mountain range that formed approximately 7 million years ago, and is composed of a series of rhyolitic lava flows. The siliceous rhyolite provides a source of silica that can form crusts of glassy hyalite opal (opal-AN) on the surface of the rhyolite, and is often brightly fluorescent due to uranium content.

This specimen is an excellent example of hyalite opal on rhyolite matrix from the Thomas Range and consists of a transparent crust of bubbly hyalite on a light brown/grey colored rhyolite matrix. The hyalite opal fluoresces bright green under shortwave UV and dimmer green under longwave UV. Due to its uranium content, this specimen emits a low level of radioactivity at approximately 200 cpm (at 1 cm).

Originally posted by Chris Clemens on Nature's Rainbows in 2017.