Sphalerite is a zinc-sulphide with varying content of iron, chemical formula (ZnFe) S. When low in iron its crystals have a yellow to brown luster. 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. The brassy yellow-gold crystals interspersed in this sample are of the mineral Chalcopyrite, a copper iron sulfide (CuFeS2) Crystal system is tetrahedral scalenohedral, and the crystal habitat may range from amorphous to tetrahedral, often modified by scalenohedral faces. Chalcopyrite is a major ore of copper. Sample is from Madan in Bulgaria, actual width 40 mm.