Prehnite got its name from Colonel Hendrik von Prehn , an 18 th century Boer who came across the mineral at the Cape of Good Hope in South Africa. In 1788 the stone was declared a distinct mineral type. The appearance of prehnite is often likened to a bunch of grapes, because it often found in crystal clusters. The mineral is sometimes referred to as the Cape Emerald , because of the crystals common green varieties. Some prehnite crystals are pleochroic - they may alter in colour, when viewed from different angles, or when the angle of the light source changes. And with several prehnite crystals, a phenomenon called chatoyancy may occur. This happens when the surface of a prehnite crystal, produces an optical effect that resembles a contracted pupil of a cat's eye. Such specimens are highly prized for their optical irregularities. Individual crystals are quite rare amongst prehnite , as they are usually found in clusters . If they do occur they are usually short, tabular , and prismatic with curved faces.

more...