Prehnite, also known obsoletely as aedelite, chiltonite, coupholite and edelite or under the trade names Cape Chrysolite and Cape Emerald is a common mineral from the mineral class of "silicates and germanates" with the chemical composition Ca2Al[(OH)2|AlSi3O10]. Chemically speaking, prehnite is therefore a calcium aluminum silicate with additional hydroxide ions.
The mineral crystallizes in the orthorhombic crystal system and usually develops tabular to prismatic crystals, but also occurs in the form of leafy, fan-shaped or clustered to spherical mineral aggregates. In its pure form, prehnite is colorless and transparent. However, it can also appear white due to multiple light refractions caused by lattice structure defects or polycrystalline formation and can take on a grey or yellowish to greenish color due to foreign impurities. Prehnite cat's eyes are also known.
Prehnite is the first known mineral to be named after a person. The Dutch baron and colonel Hendrik von Prehn (1733-1785) brought back some samples from the Cape Province (Cape of Good Hope) in 1783 and had them analyzed by Abraham Gottlob Werner. He recognized the material as a new type of mineral and named it after its finder.
The exact type locality today is the Karoo dolerite deposit near Cradock in the Eastern Cape province of South Africa.