Garnet

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Garnet
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General
CategoryNesosilicate
Formula
(repeating unit)
The general formula X3Y2(SiO4)3
Crystal systemIsometric
Crystal class
Space groupIa3d
Identification
Colorvirtually all colors, blue very rare
Crystal habitRhombic dodecahedron or cubic
CleavageIndistinct
Fractureconchoidal to uneven
Mohs scale hardness6.5–7.5
Lustervitreous to resinous
StreakWhite
Specific gravity3.1–4.3
Polish lustervitreous to subadamantine
Optical propertiesSingle refractive, often anomalous double refractive
Refractive index1.72–1.94
BirefringenceNone
PleochroismNone
Ultraviolet fluorescencevariable
Other characteristicsvariable magnetic attraction
Major varieties
PyropeMg3Al2Si3O12
AlmandineFe3Al2Si3O12
SpessartineMn3Al2Si3O12
AndraditeCa3Fe2Si3O12
GrossularCa3Al2Si3O12
UvaroviteCa3Cr2Si3O12

Garnets are gemstones that are red, but rarely blue, in color. They are nesosilicates. Their hardness on the Mohs Hardness Scale is around 7.