Prehnite

Prehnite is a calcium aluminum silicate with the chemical composition Ca2Al(AlSi3O10)(OH)2. It generally occurs as volcanic cavity infillings, and typically is a shade between yellow and green, however colorless and blue specimens are not unheard of. The stone has a soft, velvety look to it.

Prehnite as a species seems to rarely form what we would call "sharp, discrete crystals" as opposed to gorgeous ball-like aggregates and rolling plates. When they do rarely form as distinct crystals, they will have a square-shaped cross-section.

It was first described in 1788 in Karoo Dolerites Cradock, Eastern Cape Province, South Africa. Some of the best gem quality Prehnite hails from Australia.