Marcasite: Gemstone Information
Marcasite is the orthorhombic form of FeS2 (pyrite, q.v. is the cubic dimorph. Marcasite crystals are typically tabular but may be pyramidal or prismatic and may show curved faces. Fine-grained masses may be globular or resemble cockscombs. The hardness is 6–6.5 and the SG 4.88–4.92. The colour is tin-white on fresh surfaces then pale bronze-yellow darkening on exposure with an iridescent tarnish. The lustre is metallic.The jeweller’s ‘marcasite’ is typically pyrite. Marcasite occurs in sedimentary environments formed characteristically under low temperature,highly acidic conditions. It is abundant throughout the world.