Merlin diamond 104 ct. Click to enlarge image
Diamond crystal. 104 ct. Merlin, Northern Territory Image: Paul Ovendon
© Australian Museum

Diamond consists of only one element, carbon, although it commonly contains traces of nitrogen. It crystallises in the cubic system. The octahedron is a common form but many mixed forms occur as a result of formation and growth conditions. Twinned crystals (macles) are often seen.

Diamond Physical properties

  • Chemistry: Carbon
  • Hardness: 10
  • Refractive Index: 2.417
  • Specific gravity: 3.52
  • Lustre: adamantine

Diamond’s outstanding properties make it an excellent gemstone. It has:

  • The greatest hardness of any natural substance (10 on Mohs' scale). This also gives it many industrial applications.
  • A high refractive index which, combined with its superior hardness and lustre, makes a brilliant finished stone with a high polish.
  • Strong dispersion of white light, filling faceted stones with spectral colours ('fire').

Diamonds increase in value with 'whiteness', clarity and size. However, fancy colours, such as blue, green, yellow, orange, pink and red are extremely rare and therefore very valuable.

Dimonds in Australia

Australia has been a diamond producer since the nineteenth century. The largest diamond found so far in Australia was mined at the Merlin Mine, Northern Territory, in 2003 and weighed 104.73 carats.

Australia’s largest and most economic diamond deposit has been the Argyle Mine, in the East Kimberley region of Western Australia. It started production in the early 1980s and was, for some years, the world’s largest diamond mine by volume. It produces a large quantity of stones each year for the international market. Most are used in industry but approximately 5% are gemstone quality.

The Argyle mine is unique in producing a dependable supply of rare pink to red diamonds.

Diamonds have also been found in the Ellendale area of Western Australia.

New South Wales

Numerous diamond occurrences have been reported in New South Wales and some have resulted in mining. Most consistently mined has been the Copeton/ Bingara area of the New England tableland in northern New South Wales. Diamonds were discovered at Bingara in 1872 by gold prospectors and the area has been worked intermittently since then. The most productive mine was the Monte Christo.

In the same year, tin miners found diamonds at Maid’s Creek, near Copeton. The next year, many more were found and Copeton became the richest diamond field in New South Wales. Basalt-covered deep leads in the area were mined intermittently until 1922, with the Star of the South being the most productive mine.

Since the 1970s there has been further diamond exploration and test mining in the Copeton/Bingara area. Stones from Copeton and Bingara are generally yellow or colourless, small and highly resorbed, giving them a glossy appearance. They contain internal dislocations (gnaats) that result in different directions of hardness. In the past, this made the stones difficult to cut but today's technology has eliminated this problem.

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