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Originally and strictly, common crystallized iron pyrites, an iron sulphide mineral; later the word became a misnomer for iron disulphide (pyrite or white iron pyrites) that is of the same chemical composition and resembles it but is of different structure and lower specific gravity.
True marcasite (which is almost white, resembling pale bronze) was used in jewelry by the Greek and the Incas, but the substitute pyrite was used extensively in Europe from the 18th century onwards, especially in France. Marcasite and pyrite have been imitated with glass, cut steel, and plastic, but are readily distinguishable from these, although not from each other. Having been sometimes set in a marquise ring, it has occasionally been erroneously spelled 'marquisite'. Marcasite is usually set in silver (sometimes pewter), and is generally rose cut or mounted in pavé setting to enhance it sparkle, due to surface reflection rather than internal light. Some cheap examples are set in rhodium-plated mounts mad eof a base metal, sometimes set with paste and enamelled. It has been sometimes referred to incorrectly as fool's gold, which is a term for pyrite. The term 'marcasite' is today often loosely applied in the trade to cut steel or even any white metal cut with facets.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson