Mineral, KNO₃; sp. gr. 1.9–2.3; hardness 2; orthorhombic; white, grey, reddish-brown, or lemon-yellow; vitreous lustre; crystals acicular, usually forms granular crusts and uneven masses; occurs in very arid conditions where vegetation is very sparse, in association with evaporite minerals of the arid desert type (e.g. soda nitre, gypsum, halite, and occasionally iodates), and little precipitation is required to wash an encrustation of nitre into hollows where it forms massive deposits. It is used as a fertilizer.