Smithsonite from Namibia.
Smithsonite is a white, yellow, green, or blue mineral, formerly known as calamine, consisting of zinc carbonate (ZnCO3). Smithsonite occurs widely, an ore of zinc formed by alteration of other zinc minerals and commonly associated with sphalerite, hemimorphite, galena, and calcite. Its crystals, which are rare, are rhombohedral in the trigonal system; it usually occurs in masses. Blue specimens from New Mexico have been used as gemstones. It is named for the British chemist James Smithson, after whom the Smithsonian Institution is also named. Hardness 4–4.5; relative density 4.4.