Hyalite opal from the Tertiary of Mexico. (public display, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA)

The yellowish- and greenish-glowing material shown above is hyalite opal (SiO2·nH2O - hydrous silica). Hyalite is also known as glass opal. It is composed of opal-AN, which refers to amorphous opal having a network structure similar to silica glass, rather than opal consisting of silica gel spheres/colloids (most opal is the latter type).

Hyalite is normally clear/colorless. Under incandescent lighting, this Mexican material is clear to extremely pale yellow. The greenish coloration is a type of luminescence - fluorescence. Under ultraviolet light (UV light; black light) and in direct sunlight and in indirect sunlight and in blue laser pointer light, this hyalite glows greenish.

Published research has shown that the greenish luminescence is from ultraviolet excitation of uranyl impurities in the hyalite opal (Fritsch et al., 2015a, 2015b). Uranyl is a UO2 +2 ion - uranium dioxide with a double positive electric charge. This Mexican hyalite is indeed slightly radioactive, but the radiation levels do not pose a health risk (Fritsch et al., 2015b). Several uranium-bearing minerals have been identified in the opal's host rocks, including meta-autunite, haiweeite, uranophane, meta-uranospinite, and others (Fritsch et al., 2015a).

This specimen is luminescent botryoidal hyalite opal on a devitrified rhyolitic volcanic tuff matrix. This new opal variety, discovered in 2013, has been nicknamed "electric opal".

Geologic context & age: Sierra Madre Occidental Volcanic Province, mid-Tertiary

Locality: undisclosed site on a high mesa in Zacatecas State, central Mexico

Info. mostly synthesized from:

Fritsch et al. (2015a) - Green-luminescing, gem hyalite opal from Zacatecas, Mexico. 34th International Gemmological Conference, August 2015, Vilnius, Lithuania: 101-104.