Garnet

Garnet refers to a group of related silicate minerals that share a common crystal structure and general formula A3B2(SiO4)3. In crystallography, garnets are isometric and commonly form dodecahedra. The group includes end-members such as pyrope (Mg3Al2Si3O12), almandine (Fe2+3Al2Si3O12), spessartine (Mn3Al2Si3O12), grossular (Ca3Al2Si3O12), andradite (Ca3Fe2Si3O12), and uvarovite (Ca3Cr2Si3O12). These can mix in solid solution, yielding the pyralspite series (pyrope-almandine-spessartine) and the grandite series (grossular-andradite).

Color ranges from red to orange, yellow, green, and brown; rare pink or colorless varieties exist. Color is caused by impurities such as Fe, Mn, Cr, or V. Garnets are typically transparent to translucent and, due to their cubic symmetry, are isotropic, showing no birefringence.

Garnet forms during metamorphism in schists, gneisses, granulites, and some contact aureoles, and occurs in certain igneous rocks as well as ultramafic rocks. Detrital garnet grains are common in sedimentary deposits and can persist through erosion and transport.

Physical properties include a Mohs hardness of about 6.5–7.5 and a specific gravity of roughly 3.5–4.3. Garnet crystals have a vitreous luster and a typically conchoidal or uneven fracture; there is no true cleavage. Refractive indices vary with composition, generally around 1.72–1.94.

Name origin: from Latin granum meaning seed or grain. Uses include gemstone material in jewelry and as industrial abrasives for cutting and grinding.