AVENTURINE or AVANTURINE, a variety of quartz spangled with scales of mica or haematite. Most aventurine quartz is of yellow or brown colour, but a green variety containing scales of the chrome-mica, fuchsite, is also known. The name aventurine is also applied to certain iridescent felspars. The principal of these is the oligoclase occurring in gneiss at Tvedestrand in southern Norway. The brilliant spangled appearance of such felspars is due to microscopic enclosures, the colours in reflected light being the interference colours of thin films. In most cases the r>?flecting lamellae are haematite, oriented always after simple crystal forms causing "aventurization" on the planes oo1 and oTo. Aventurine felspar is also known as Sunstone, but aventurization is not confined to oligoclase, occurring also in orthoclase and labradorite (q.v.).