Prehnite

Prehnite derives its name from Colonel von Prehn, who discovered the gemstone in South Africa in 1788 and brought it back with him to Europe. Characteristically known as an oily mint green, prehnite may also appear as yellowish to a light brown. 

The majority of prehnite on the market occurs naturally as a translucent mass. Columnar and tabular crystals are rare, as the stone occurs more usually as aggregates of barrel-shaped crystals or as botryoidal masses. Some pale yellowish brown prehnite is fibrous enough to be cut as a cabochon, and may show a cat's eye effect. 

Very occasionally prehnite is pleochroic; a rare phenomenon which increases the value of the stone. While some prehnite is translucent and clean, other material can be full of flaws and inclusions such as fractures, black tourmaline acicular inclusions and bright copper specks.

Individual gem-quality transparent crystals are rare and usually small, remaining in the domain of collectors. Pale green masses are found in Scotland, dark green or greenish brown masses in Australia, and aggregates of crystals in France. 

Sources:
Judith Crowe, The Jeweller's Directory of Gemstones (London: A&C Black, 2006)
Cally Hall, Gemstones (London: Dorling Kindersley, 1994)
Jaroslaw Bauer and Vladimir Bouska, A Guide in Colour to Precious & Semiprecious Stones (London: Octopus Books, 1983)