Hessonite Click to enlarge image
Garnet 2.50 carats Origin - Harts Range, N.T., Australia Image: Stuart Humphreys
© Australian Museum

Garnet is the name given to a group of complex silicate minerals with a similar chemical formula, similar physical properties and a common crystal system. Five of these minerals, almandine, pyrope, spessartine, grossular and andradite, are used as gemstones. Garnet forms in the cubic system and is common in Australian metamorphic rocks. Localities for gem quality garnets are more restricted.

Garnet Physical properties

  • Chemistry: Aluminium silicate
  • Hardness: 6.5 - 7.5
  • Refractive Indices: 1.730 - 1.888
  • Specific gravity: 3.65 - 4.20
  • Lustre: vitreous

Garnet in Australia

Grossular garnet is found in the Harts Range, Northern Territory. here it occurs in its pure, colourless form and with the addition of iron to colour it yellow, orange and red. The yellow and orange colours are known as hessonite garnets.

New South Wales

The red pyrope-almandine series of garnets, in which one type gradually changes into the other, is found at many Australian gem localities. Almandine is the iron-rich end member, while pyrope is magnesium rich. An attractive, in-between type has been given the name rhodolite. Gem quality garnets are found in the New England area and at other localities in New South Wales.

Spessartine is a manganese garnet, coloured from orange to red. Fine crystals occur at Broken Hill, New South Wales.

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