A potassium nitrate mineral with chemical composition KNO₃. Niter crystallizes in the orthorhombic system, generally in thin crusts and delicate acicular crystals; it also occurs in massive, granular, or earthy forms. It has good cleavage in three directions; fracture is subconchoidal to uneven; it is brittle; hardness is 2 on Mohs scale; specific gravity is 2.109. The luster is vitreous, and the color and streak are colorless to white.