Silicate mineral Al3(Mg,Fe)2 [Si5AlO18] which can be iron-rich and is a member of the cyclosilicate group; sp. gr. 2.5–2.8; hardness 7; orthorhombic; dark blue or greyish-blue, translucent to transparent; vitreous lustre; crystals rare, prismatic or pseudo-hexagonal, but usually occurs massive; cleavage imperfect {010}, parting {001}; occurs in aluminous rocks that have been subjected to thermal or regional metamorphism, in hornfels, schists, and gneisses, in association with andalusite, spinel, quartz, and biotite. Fine dark blue examples are used as gemstones. It is named after the 19th-century French geologist P. L. A. Cordier (1777–1861).