Prehnite with tourmaline tumbled from Mali.

The mineral Prehnite is a calcium-aluminum-silicate with the chemical formula Ca2Al2Si3O10 (OH). The mineral belongs to the fylosilicaten. The transparent to translucent, colorless, white, gray or yellow to yellow-green prehnite has a glass to pearl luster, a white stripe color and cleavage is clear according to the crystal surface [001]. Prehnite has an average density of 2.87, and the hardness is 6 to 6.5. The crystal system is orthorhombic and the mineral is not radioactive. The mineral prehnite is named after the Dutch colonel Hendrik von Prehn (1733 - 1785), who discovered the mineral in South Africa. Prehnite was the first mineral, which was named after a person. Prehnite is a mineral that occurs fairly common in hydrothermal weathered rocks. Although it is not a zeolite, it is often found in the vicinity of zeolites. It is among others found in granites, gneisses and syenieten. It is also indicative mineral for the low-grade metamorphic facies prehnite-pumpellyiet. The type of location is Haslach, Harzburg Oberstein, Germany.