Garnet

The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" comes from 14th century Middle English word gernet meaning 'dark red', from the Latin granatus granatus coming from granum (grain , seed) + suffix "atus" , possibly a reference to "mela granatum" or even "pomum granatum", a plant whose abundant vivid red arils contained in the fruit are similar in shape, size, and color to some garnet crystals.

Six common species of garnet are recognized by their chemical composition. They 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-spessarite and uvarovite-grossular-andradite.

Source: en.wikipedia.org/wiki/Garnet

Garnet Pictures

Garnet in rhombic dodecahedron form

Garnet Properties

Chemical Composition: 
Basic Formula: A3B2C3 where: A = Ca, Mn, Mg, Fe; B = Al, Fe, Ti, V, Cr; C = (SiO4)
Classification / Type: 
This is a major isomorphous group of gemstones. This group is divided into 2 sub-groups of 3 species each. Due to the strong isomorphism between species, a very high percentage of garnets are classified as Intermediate species.
  1. Pyralspite Series
    1. Pyrope: Mg3Al2(SiO4)3
    2. Almandine: Fe3Al2(SiO4)3
    3. Spessartite: Mn3Al2(SiO4)3
  2. Ugandrite Series
    1. Grossular: Ca3Al2(SiO4)3
    2. Andradite: Ca3Fe2(SiO4)3
    3. Uvarovite: Ca3Cr2(SiO4)3
Colors / Varieties: 
  • Idiochromatic Garnets
  • Allochromatic Garnets
    • Pyrope: Intense red color, orangish red, violet-red
    • Grossular: Orange to yellowish orange, reddish orange to brownish orange
    • Andradite (Demantoid): Green, yellowish-green
Crystal System / Forms: 
Cubic System / Dodecahedron, Trapezohedron or a combination of varying cubic forms.
Cleavage / Fracture: 
None but when present is imperfect and poor cleavage parallel to dodecahedral faces / Sub-conchoidal to uneven fracture
Optic Character: 
Isotropic; may exhibit A.D.R. reaction. Red, purplish red, brown and yellowish brown may show A.D.R. (S.R.) or a misguiding D.R. reaction.
Lustre: 
Vitreous to Adamantine
Geological Occurrence: 
In metamorphic and igneous rocks; also as alluvial deposits. Also in kimberlite, peridotite, serpentine.
Cuts & Uses: 
Facetted, cabochon, beads, carvings, etc.