A mineral sorosilicate having the composition Zn₄Si₂O₇(OH)₂ · H₂O; an ore of zinc. It crystallizes in the orthorhombic system, pyramidal class, and thus the prismatic crystals have different forms at top and bottom (see illus.). Also, as a result of the symmetry, the vertical axis is polar and crystals display the properties of pyroelectricity and piezoelectricity. There is perfect prismatic cleavage. Botryoidal and staclactic aggregates, showing a crystalline surface and frequently impure, are common. Crystals are usually colorless and the aggregates white, but in some cases there are faint shades of green, yellow, and blue. The mineral has a vitreous luster, a hardness of 4½ to 5 on Mohs scale, and a specific gravity of 3.45. Hemimorphite frequently resembles the zinc carbonate, or smithsonite, but can be distinguished by the reaction with hydrochloric acid. Hemimorphite is slowly soluble but smithsonite effervesces in cold acid.