Opalized Volcanic Tuff, Burro Creek, Arizona
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 SW UV, and dim green under LW UV. This piece has a single cut surface that provides an excellent view of the interior. The size of this specimen is 11.2 x 10.6 x 5.9 cm, and it weighs 430 grams.
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.