Garnet
| Garnet | |
|---|---|
 |
|
| General | |
| Category | Nesosilicate |
|
Formula (repeating unit) |
The general formula X3Y2(SiO4)3 |
| Crystal system | Isometric |
| Identification | |
| Color | virtually all colors, blue very rare |
| Crystal habit | Rhombic dodecahedron or cubic |
| Cleavage | Indistinct |
| Fracture | conchoidal to uneven |
| Mohs scale hardness | 6.5–7.5 |
| Luster | vitreous to resinous |
| Streak | White |
| Specific gravity | 3.1–4.3 |
| Polish luster | vitreous to subadamantine |
| Optical properties | Single refractive, often anomalous double refractive |
| Refractive index | 1.72–1.94 |
| Birefringence | None |
| Pleochroism | None |
| Ultraviolet fluorescence | variable |
| Other characteristics | variable magnetic attraction |
| Major varieties | |
| Pyrope | Mg3Al2Si3O12 |
| Almandine | Fe3Al2Si3O12 |
| Spessartine | Mn3Al2Si3O12 |
| Andradite | Ca3Fe2Si3O12 |
| Grossular | Ca3Al2Si3O12 |
| Uvarovite | Ca3Cr2Si3O12 |
Garnets (pronunciation: /ˈɡɑːrnᵻt/) are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.
All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different species are pyrope, almandine, spessartine, grossular (varieties of which are hessonite or cinnamon-stone and tsavorite), uvarovite and andradite. The garnets make up two solid solution series: pyrope-almandine-spessartine and uvarovite-grossular-andradite.
The word garnet comes from the 14th‑century Middle English word gernet, meaning 'dark red'. It is derived from the Latin granatus, from granum ('grain, seed'). This is possibly a reference to mela granatum or even pomum granatum ('pomegranate',Punica granatum), a plant whose fruits contain abundant and vivid red seed covers (arils), which are similar in shape, size, and color to some garnet crystals.
Garnet species are found in many colors including red, orange, yellow, green, purple, brown, blue, black, pink, and colorless, with reddish shades most common.
Garnet species' light transmission properties can range from the gemstone-quality transparent specimens to the opaque varieties used for industrial purposes as abrasives. The mineral's luster is categorized as vitreous (glass-like) or resinous (amber-like).
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Wikipedia