Chemical composition -- Iron sulfide. Often confused with pyrite or fool's gold, a slightly denser form of iron sulfide that crystallizes in the isometric (cubic) system.
Color -- Whitish to brassy yellow. Gradually darkens on exposure to air.
Optics -- Opaque, metallic luster.
Durability -- Hardness 6-6.5.
Crystal structure -- Orthorhombic.
Specific Gravity -- 4.85.
Sources -- Many sources throughout the world. Marcasite is often faceted as small rose cuts and was popular in Victorian jewelry.