Prehnite, also known as aedelite, chiltonite, coupholite and noblerite 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]. Prehnite is thus chemically a calcium-aluminium silicate with additional hydroxide ions.

The mineral crystallizes in the orthorhommy crystal system and usually develops table to prismatic crystals, but also occurs in the form of leafy, fan-shaped or trusty to spherical mineral aggregates. In pure form, Prehnite is colourless and transparent. Due to multiple refraction of light due to lattice construction defects or polycrystalline formation, however, it can also appear white and assume a grey or yellowish to greenish color by external additions. Prehnite cat eyes are also known.

Prehnite is the first known mineral named after a person. The Dutch baron and colonel Hendrik von Prehn (1733-1785) brought 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 mineral species and named it after his finder.

The exact type location is now the Dolerite deposit of Karoo near Cradock in the eastern cape province of South Africa.