"Fishtail" Selenite / Gypsum Specimen - Sierra Mojada, Mexico

Selenite, a type of gypsum, is a hydrous calcium sulfate (CaSO4•2H2O) that forms monoclinic crystals with prismatic or bladed shapes and distinct rhombic appearance. It can also be found as arrowhead-shaped twins with beveled edges. While pure selenite is colorless or white, impurities can result in shades of pale gray, yellow, red, and brown. With a hardness of 1.5-2.0, selenite is transparent to translucent and has a vitreous to pearly luster. It has a specific gravity of 2.3-2.4 and perfect cleavage in one direction. This mineral is commonly found in chemical-sedimentary, marine-evaporite formations such as sandstone, limestone, and rock salt.

Locality: Sierra Mojada, Sierra Mojada Municipality, Coahuila, Mexico

Weight: 416g

Size: 11cm x 10cm x 8cm