Garnet is a more complex orthosilicate (than olivine, for example) in which the SiO4 tetrahedra are still independent.

Garnets have the general chemical formula A3B2Si3O12, where A is a divalent cation (Fe2+, Ca2+, Mg2+, Mn2+) and B is a trivalent cation (Fe3+, Al3+, Cr3+). The end-members pyrope, almandine, and spessartine form one solid solution series, while the end-members grossular, andradite and uvarovite form another.

Although valued as a gem stone, garnet is generally of low monetary value because of its relatively common occurrence.

Garnet is commonly found in highly metamorphosed rocks and in some igneous rocks. They form under the same high temperatures and / or pressures that form those types of rocks. Garnets can be used by geologists to gauge the temperature and pressure under which a particular garnet-bearing rock formed.


garnet

Chemical composition - Fe3Al2Si3O12 (almandine)
Hardness - 6.5-7.5
Specific gravity - 3.6-4.3
Transparency - Transparent to opaque
Colour - Variable - most commonly red, reddish brown
Streak - White
Lustre - Vitreous to resinous
Cleavage/fracture - Non-existent / conchoidal
Crystal habit/mode of occurrence - Prismatic (12-sided rhombic, 24-sided trapezoidal) / granular, massive

Other specimens - Click the thumbnails to enlarge