Green-luminescing hyalite opal from zacatecas, Mexico

Green daylight-fluorescing hyalite opal was discovered in Mexico's Zacatecas State in 2013. It occurs as botryoidal coatings commonly up to 1 cm thick along fractures and cavities in a poorly welded rhyolitic tuff. The material possesses spectacular colour behaviour, appearing near-colourless or pale-tomoderate yellow in incandescent light and vivid greenish yellow to yellowish green in indirect sunlight. The daylight-induced green luminescence is related to the presence of trace amounts of the uranyl molecule (UO₂)²⁺. Several dozen high-quality transparent gemstones in the 1-6+ ct range have been faceted from this material so far, in addition to hundreds of smaller stones. The gems typically contain two-phase fluid inclusions and curved deposition marks, and they show bright interference colour patterns between crossed polarizers. This hyalite is not porous, and contains approximately 2.7 wt.% H₂O, which is remarkably low among gem opals. The radioactivity of the opal is within background levels, and does not pose any health concern.