Sphalerite embedded in a double ended calcite crystal. Sphalerite is zinc-sulphide mineral with varying content of iron, chemical formula (ZnFe)S. When low in iron its crystals have a yellow to brown lustre. With more iron it is an opaque, dark metallic-grey, as in this sample, and then sometimes referred to as marmatite. Crystals euhedral, often with lamellar surface, crystal system is isometric. Sphalerite is the most common ore of zinc. Provenance is the Elmwood mine in Carthage, Tennessee, USA and actual width of sample is 180 mm.