Prehnite

Prehnite is a low-temperature mineral deposited from groundwater in cavities of basaltic rocks together with zeolites; it also forms hydrothermally in cavities of acid igneous rocks and in serpentine rocks. Its type locality is in the Karoo dolerites near Cradock, Eastern Cape, South Africa. Sources include New Jersey and Virginia (USA), Asbestos in Quebec (Canada), Scotland and Australia (facetable material), as well as France, Italy, Austria, Germany, Russia, South Africa, Pakistan and New Zealand.

On a refractometer prehnite usually gives a shadow edge near 1.63; its specific gravity is about 2.88–2.94 for gem material. It shows no pleochroism and may glow dull brownish yellow under UV light. Translucent material can look soft and velvety; chatoyant (cat's-eye) stones have been reported.

Prehnite was named in 1788 by Abraham Gottlieb Werner in honor of the Dutch colonel Hendrik von Prehn, who is credited with discovering it in 1774 at the Cape of Good Hope and who brought the first specimens to Europe. It has been claimed to be the first mineral named after a person.