A naturally occurring rhombohedral zinc carbonate (ZnCO₃), with a crystal structure similar to that of calcite (CaCO₃). The structure consists of alternating layers of zinc ions (Zn²⁺) and carbonate ions (CO₃^2-) oriented perpendicular to the c crystallographic axis. Smithsonite has a hardness on the Mohs scale of 4¹/₂, has a specific gravity of 4.30–4.45, and exhibits perfect rhombohedral cleavage. Optically, smithsonite is uniaxial negative, which means that it has a single optic axis and two principal refractive indices referred to as the ordinary refractive index and the extraordinary refractive index. Smithsonite is classified as optically negative because the ordinary refractive index is greater than the extraordinary index. (N_o > N_e).