Mineral, Al₂O₃; sp. gr. 3.9–4.1; hardness 9; trigonal; the two main varieties blue and green, but can be yellow, or brown to almost black, and transparent; adamantine to vitreous lustre; crystals usually rough and barrel-shaped, tapering, and also flat and tabular; no cleavage, partings {0001}, {0112⁻}; occurs in silica-poor rocks such as nepheline syenites and undersaturated (see silica saturation) alkali igneous rocks, in contact aureoles in thermally altered alumina-rich shales or limestones, in aluminous xenoliths found within basic igneous rocks in association with spinel, cordierite, and orthopyroxene, in metamorphosed bauxite deposits and in emery deposits, and in alluvial deposits because of its hardness and resistance to abrasion along with muscovite, hematite, and rutile. Flawless crystals are the gemstones blue sapphire, red ruby, and green oriental emerald. The main use of corundum is based on its hardness. It is made into grinding wheels and discs, emery paper, and powders for grinding and polishing.