A mineral with composition ZnO (zinc oxide). It crystallises in the hexagonal system with a wurtzite-type structure. Thus its principal axis is polar and different forms appear at top and bottom of crystals. Such crystals are rare and the mineral is usually massive. Cleavage is prismatic. Its hardness is 4 and its specific gravity 5.6. The mineral has a subadamantine lustre and a deep-red to orange-yellow color. The colour of the mineral is believed to result partly from the presence of manganese and partly from defects in the crystal structure. Zincite is rare except at the zinc deposits at Franklin and Sterling Hill, New Jersey. There, associated with franklinite and willemite, it is mined as a valuable ore of zinc.