Cadmoselite
Cadmoselite is a rare cadmium selenide mineral with chemical formula CdSe. Cadmoselite crystallizes in the hexagonal system and occurs as black to pale grey opaque crystals and grains.[1][2]
It was first described in 1957 for an occurrence in Tuva. The mineral occurs as interstitial grains in sandstone formed under reducing alkaline diagenetic conditions.[1][2]