Bauxite is a mixture of several minerals, including gibbsite, diaspore, boehmite, haematite, kaolinite and opal. Its variable composition gives it variable characteristics. Bauxite often appears as platy, nodular, oolitic or pisolitic aggregates. The colour variation is also significant, which can range from white, light yellow, and red to reddish brown. The streaks are usually white. It has a dark or earthy lustre, opaque diaphaneity, a specific gravity of 2.3–2.7, a Mohs hardness of 1–3, and uneven fractures. It is mainly formed in igneous and metamorphic rocks with high aluminium contents. Under hot and humid conditions, rain will leach the silicate from the rock, leaving bauxite minerals behind as the residual material.