Opal var. Hyalite with Chalcedony

Tepetate is a classic locality in San Luis Potosí, best known for its topaz but also for unusual hyalite opal. The opal commonly forms clear, rounded or bubbly growths on chalcedony, creating a nice contrast between the glassy hyalite and more opaque silica. Some specimens from the area show strong green fluorescence under shortwave UV.