Diamond is a mineral composed of carbon with an isometric crystal structure, the most common form being octahedral. It is universally known for its spectacular gem qualities — high refractive index (2.42), exceptional colour dispersion, and brilliant lustre on cut faces. Diamond can be colourless, or coloured blue (due to small quantities of boron), yellow (due to small quantities of nitrogen), green (caused by radiation), pink, brown or black. Diamonds of gem quality have for centuries been cherished as symbols of love and eternity, captured in that memorable phrase, "a diamond is forever".
Diamond is the hardest substance known (10 on the Mohs scale) which makes it useful in industrial applications involving grinding, cutting and polishing. It is the only material capable of accurately and economically cutting some modern materials such as ultrahard ceramics. Its unique electrical, optical and thermal characteristics are used in special lenses, heat sinks in electronic circuitry, and in many advanced technologies. Natural industrial diamonds are those discarded as being less than gem quality due to inferior colour, size or other defects. The first synthetic diamonds were made by General Electric in the USA in 1954, and the consumption of synthetics now greatly exceeds that of natural industrial diamonds, which account for only 3% of all industrial diamonds used.
Diamond size is traditionally measured in carats (ct), 1 ct being 0.2 gm.
The industry has been on a rollercoaster ride since the financial crisis of 2008, with prices dipping, and rebounding. In 2012 global rough diamond sales were US$14.8 billion, global retail sales of diamond jewelry were US$72 billion. World production of a natural rough diamond in 2012 was 128 million carats, with Australia's production share at 17% or 21.8 million carats. 70% of the world's diamond resources are located in Africa and Russia, with the total global resource in 2012 estimated at 2.3 billion carats. Australia's 2012 resource was ~ 165 million carats.
The primary source of most natural diamonds is kimberlite, a hybrid ultramafic igneous rock composed principally of olivine with lesser amounts of phlogopite, diopside, serpentine, calcite, garnet and other minerals. The rock type is named after Kimberley in South Africa where it was first identified in 1870.
These rocks originate ~150 km below the Earth’s surface and act as vehicles for transporting diamonds to the surface from deep in the crust. They typically occur as vertical pipes ranging from less than a metre to almost 1 km across. The discovery of diamonds in lamproite, a related rock type, in Western Australia in 1979 has led to further exploration for lamproite-hosted deposits. Both kimberlite and lamproite belong to a broader category of igneous rocks called lamprophyres. It must be noted that not all kimberlite and lamproite bodies contain diamonds. Diamonds are also found in placer deposits some distance from the eroded kimberlitic source rocks.
Diamond occurrences in South Australia
South Australia is considered highly prospective for diamonds. Macrodiamonds (many of which are gem quality), microdiamonds and diamond indicator minerals have been found in the State, and there are widespread occurrences of kimberlite and lamprophyre associated with cratonic fractures and lineaments.
Three sites in South Australia have each recorded more than 100 diamonds:
Diamonds were first found at Echunga near Adelaide by gold panners in 1859, and reports of up to 200 diamonds including 50 saleable gems were recorded through to 1900. Since then only one has been found, a 0.9 ct stone in 1987.
In the Springfield Basin, 128 diamonds and fragments were recovered in the mid-1980s from Permian basal conglomerate. In 1998, an additional 56 diamonds were recovered from renewed exploration trenching, the largest being 3.5 mm and weighing 0.34 ct. Fresh indicator minerals, particularly garnet with a kelyphytic rind, suggest that the kimberlite source is relatively close to the basin.
At Eurelia, 140 microdiamonds have been found associated with kimberlite dykes and one pipe; this is the first area in the State where diamonds have been retrieved from source rocks.
There are many other localities in South Australia where lesser numbers of diamonds have been found, including Edwards Creek with eight yellow microdiamonds totalling 0.75 ct. Localities of diamond finds and occurrences of lamprophyric rocks throughout the State are shown on (Fig 1, (pdf ~ 75kb)).
Flinders Diamonds was successfully floated on the Australian Stock Exchange on the 22nd of February, 2002 with funds raised for exploration in several tenements including the Springfield project, Flinders Ranges where 90 indicator mineral anomalies have been located during previous exploration. Other targeted areas include the Adelaide Hills and south of the Peake and Denison Ranges where several diamonds and numerous kimberlitic indicators have been found.
Diamond source rocks
Lamprophyric (including kimberlitic) rocks are widespread in South Australia. Significant areas include:
- Eyre Peninsula. At least eight kimberlite pipes and dykes of probable Jurassic age have been defined by recent exploration, including an altered monticellite kimberlite pipe and dyke complex north of Cowell. Kimberlite occurrence is probably controlled by deep east–west-trending fractures of the Polda Basin. In July 2001, Tawana Resources NL reported three microdiamonds from loam sampling on Flinders Island off Elliston on Eyre Peninsula following reports of abundant fresh kimberlite indicator minerals found in previous loam sampling. Tawana is confident of at least one kimberlite occurring on the island, and a multi phase drilling program was initiated in November 2001 and continues into 2002.
- Eurelia. A swarm of en echelon Jurassic kimberlitic dykes up to 30 m wide and 1 km long extend over a distance of 20 km.
- Terowie. Manunda five clusters of dykes and pipes occur in a 45 x 25 km zone.
- Truro. work by PIRSA Mineral Assessment Branch has defined >20 lamproite dykes and a diatreme which are probably of Ordovician age.
- Radium Hill. Several Ordovician phlogopitic lamprophyre dykes.
- Port Augusta. Three micaceous kimberlite sills.
- Mulgathing, northwest of Tarcoola. Numerous basic to ultrabasic mica-peridotite plugs and sills with kimberlitic affinities.
- Abminga. Drilling at Abminga has recovered crater sediments with kimberlitic affinities at several anomalies, and two small macrodiamonds have been recovered from drillcore at one location.
Townsend, I.J., Morris, B.J. and Farrand, M.G., 1994. Review of diamond resources in South Australia. South Australia. Department of Mines and Energy. Report Book, 94/34.
The Global Diamond Report 2013. Bain & Company Inc, 2013