Prehnite points from Mali, 10-15 cm in size and sold per kilogram.

The mineral prehnite is a calcium-aluminium-silicate with the chemical formula Ca2Al2Si3O10(OH). The mineral belongs to the phyllosilicates. The transparent to translucent colourless, white, grey or yellow to yellow-green prehnite has a glass to pearl lustre, a white stripe colour and the cleavage is clear according to the crystal plane [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 to be named after a person. Prehnite is a mineral that occurs fairly commonly in hydrothermally weathered rocks. Although it is not a zeolite, it is often found near zeolites. It is found in granites, syenites, and gneisses, among other places. It is also an indicative mineral for the low-grade metamorphic facies prehnite-pumpellyite. The type locality is Haslach, Harzburg and Oberstein, Germany.