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
  • Almandine: Red, purplish red, purple
  • Spessartite: Orange, yellowish orange, brownish orange
  • Uvarovite: Green, Yellowish green
  • Andradite (Topazolite): Yellow, greenish yellow
• 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

Pleochroism: 

Nil

Geological Occurrence: 

In metamorphic and igneous rocks; also as alluvial deposits. Also in kimberlite, peridotite, serpentine.

Cuts & Uses: 

Facetted, cabochon, beads, carvings, etc.