Garnet

Garnet
Garnet is a group of minerals that have been used since the Bronze Age as gemstones and abrasives. Garnets are most often seen in red, but are available in a wide variety of colors spanning the entire spectrum. The name "garnet" comes from the Latin granatus ("grain"), possibly a reference to the Punica granatum ("pomegranate"), a plant with red seeds similar in shape, size, and color to some garnet crystals.

Six common species of garnet are recognized based on their chemical composition. They are pyrope, almandine, spessartite, grossular (varieties of which are hessonite or cinnamon-stone and tsavorite), uvarovite and andradite. The garnets make up two solid solution series; 1. pyrope-almandine-spessarite and 2. uvarovite-grossular-andradite.

General
Category nesosilicates
Chemical formula The general formula X3Y2(SiO4)3
Identification
Color virtually all colors
Crystal habit rhombic dodecahedra or cubic
Crystal system Isometric
Cleavage None
Fracture conchoidal to uneven
Mohs Scale hardness 6.0 - 7.5
Luster vitreous to resinous
Polish luster vitreous to subadamantine [1]
Refractive index 1.72 - 1.94
Optical Properties Single refractive, often anomalous double refractive [1]
Birefringence None
Pleochroism only in certain rare varieties
Streak White
Specific gravity 3.1 - 4.3
Major varieties
Pyrope Mg3Al2Si3O12
Almandine Fe3Al2Si3O12
Spessartite Mn3Al2Si3O12
Andradite Ca3Fe2Si3O12
Grossular Ca3Al2Si3O12
Uvarovite Ca3Cr2Si3O12