Bournonite is a sulphosalt mineral, a sulfo-antimonite of lead and copper (PbCuSbS3). The crystals are orthorhombic, and are generally tabular in habit owing to the predominance of the basal pinacoid. Numerous smooth bright faces are often developed on the edges and corners of the dark-metallic crystals. It is a mineral formed in medium temperature hydrothermal vein deposits. It occurs associated with galena, tetrahedrite, and many other metal-bearing minerals; in this sample with beige-brown, tabular crystals of siderite. Here seen on a substrate of flaky crystalline gypsum.