Opalized Volcanic Tuff - Burro Creek, Arizona

Contributed by: Chris Clemens
Date: Apr 27th, 2026
Locality: Burro Creek, Mohave County, Arizona, USA (See on Mindat)
Size: 10.6 x 5.9 x 11.2 cm
Weight: 430 g

Description:
Light-colored, dendritic, brecciated opalized volcanic tuff (opalite) having a sponge like texture. Contains weblike veins of hyalite opal (opal-AN) that fluoresce bright green under shortwave UV, and dim green under longwave UV. This piece has a single cut surface that provides an excellent view of the interior.

Volcanic tuff is a type of rock formed by the compaction of volcanic ash, and the ash serves as a source of silica for the formation of the opal. The green fluorescence of the opal is due to the inclusion of trace amounts of uranium in the form of the uranyl ion, which is brightly fluorescent under UV.

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