Diamond facts and figures

Diamonds are the most popular of all of the world's gemstones.

They are often referred to as the "gem of gems", due to their exquisite beauty, which is largely a result of their ability to refract light. While all transparent gems refract light, the refractive index of the diamond is the highest of all gems, making them sparkle more than other stones.

Natural diamonds were formed approximately three billion years ago, 150 kilometres beneath the earth's surface.

Under very high temperatures (between 900 and 1300 degrees Celsius) and great pressure, carbon atoms combined to crystallise and grow as diamonds within rocks.

Molten lamproite and kimberlite (magma) was also forming within the Earth's upper mantle, and, subjected to similar pressures, was forced upwards through the diamond-bearing rocks. The magma partially melted the rocks and created a deep and narrow channel while carrying the diamonds towards the Earth's surface.

As the magma came closer to the surface, it exploded and erupted at the surface to form a pipe. Over millions of years the lamproite and kimberlite ash within the pipes solidified and became diamond-bearing rock known as tuff. Both lamproite and kimberlite are magnesium-rich and contain minerals rich in chromium, titanium and potassium.

To produce synthetic diamonds, pressures of more than 30,000 times atmospheric pressure and temperatures greater than 1400 degrees Celsius are required. In recent years a low pressure method known as chemical vapour deposition, has been developed which grows layers of polycrystalline diamond from gases.

Diamonds of powder size can also be formed by shock waves produced from explosions. Such production has been achieved in the laboratory and also in nature where meteorites have struck the earth.

Other key diamond facts and figures are:

• Diamond is a compact crystalline form of carbon, with the atoms arranged to form the hardest substance known. The perfect diamond crystal shape is an octahedron.
• Diamonds are measured in weight in terms of carats. One carat = 0.2 grams. The term carat originated from the carob seed, an early Arabian trading measure.
• The first known source of diamonds was in India, where they were mined from alluvial gravel more than 4,000 years ago.
• Indian stones were sent to Greece, where they were known as "adamas", from which the name diamond is derived.
• Until the early 18th century, diamonds were very rare, however in 1725 alluvial prospectors found diamonds in Brazil and more diamonds became available throughout the world.
• In 1866 the first South African diamond was found. It weighed 21 carats.
• In 1869 the 83 carat "Star of South Africa" was found. This was followed by the discovery of a number of large kimberlite pipes, which transformed the world diamond market. At the start of the 20th century, diamond pipes and alluvial deposits were found throughout Africa.
• In 1954 the Zarnitsa pipe was found in Siberia, and the former Soviet Union became a large-scale producer of diamonds.
• Synthetic diamonds began production in Sweden in 1953.
• In Australia, the first diamond was found by a gold prospector near Bathurst, New South Wales, in 1851.
• The largest diamond ever found was the Cullinan, which weighted 3106 carats (621 grams) and came from South Africa's Premier mine in 1905.
• The Cullinan diamond was cut into nine large gems and 100 smaller stones and now forms part of the British Crown Jewels.
• The second and third largest diamonds ever found are the Excelsior (995 carats) from South Africa and the Star of Sierra Leone (969 carats).
• The world's largest diamond mine was discovered in 1979 to become the Argyle Diamond Mine.
• The highest price ever paid at auction for a diamond was a 0.95 ct purplish red stone. In 1987 this sold for US$927,000/ct.
• The second highest price ever paid at auction for a diamond was a 1.92ct fancy red stone. In December 2001 this sold for more than US$860,000/ct.

Diamond properties

Diamonds are one of the most studied substances because of their unique properties. When they were first discovered it was the mineral's exceptional hardness for which it was noted. Since then numerous other special properties have seen diamond used in industrial, medical and scientific applications. The following table shows the main physical properties of diamond.

• Industrial diamonds are those which are unable to be used as gem or near-gem by virtue of their shape, colour or quality. Their application depends on their form. Those with sharp tips or edges may be used in their natural state as a dressing tool for grinding wheels or for large rock drills. Industrial diamonds of 1 mm - 3 mm in diameter may be processed to produce rounded stones for rock drilling bits. While heavily cracked or included diamonds of any size may be crushed to produce grit for saws or powder for polishing.
• Besides hardness, diamonds find application in the machining industry on account of their very low friction, generating less heat at any cutting surfaces. Also, their extremely low coefficient of thermal expansion means a diamond cutter will maintain its dimensions despite any thermal changes. While diamonds are extremely hard, they are correspondingly brittle and can be readily shattered by impact.
• Not all non-jewellery applications of diamond use industrial diamonds, with gem qualities needed for some demanding applications. The very low friction of diamonds and its hardness has seen the material fashioned into a scalpel blade and used for delicate surgery, particularly ocular surgery. The low friction means it slices through tissue easily while the high hardness ensures the edge does not deteriorate. In addition, diamond is hydrophobic, meaning it repels water, so thin slices of tissue do not stick to the blade.
• Diamonds have the lowest specific heat and the highest thermal conductivity of any solid. These attributes make diamonds the ideal candidate for heat sinks such as used for electronic chips. Such electronic applications are normally satisfied by synthetic diamonds, where better control of impurities and dimensions is possible.
• Optically a diamond is special, giving it its distinctive place in jewellery. Its high refractive index (2.42) provides it with a steep angle (24 degrees off normal) for total internal reflection. The material's dispersion (0.044) which is responsible for the 'fire' of a facetted gem, is not extraordinary with numerous other substances exhibiting higher values.
• The spectral transmission of diamonds is notable as a colourless sample is transparent from the ultra-violet through to the mid infrared. It is this characteristic which has seen diamonds used as camera windows on a Venus probe and also on infrared seeking missile noses which have to withstand erosion from rain drops. The former use was also exploiting the diamonds' resistance to acids.
• Electrically, diamond is an exceptional insulator. However in the presence of trace amounts of boron, the substance becomes a semiconductor. The electrical resistance of such conducting diamonds has an extremely high temperature dependence, which has seen the substance used as a highly sensitive thermometer detecting temperature changes as low as a millionth of a degree centigrade.
• The presence of trace impurity elements in diamond is also largely responsible for the colours in diamond. A perfect diamond crystal comprises only carbon atoms arranged in a cubic lattice structure. Such a diamond is colourless (also termed 'white'). Variations from this perfect structure in the form of impurities or structural deformations can impart colour to a diamond.
• The most common impurity in diamond is nitrogen which, being of a similar size to carbon, can be integrated into the crystal lattice. These nitrogen-bearing diamonds are termed type 1 diamonds and are by far the most abundant in nature. The nitrogen can be manifest in several forms, from isolated single N atoms (type 1b diamonds) to clusters of 2 (type1aA), 3 or 4 (type1aB) atoms. It is the presence of nitrogen that is responsible for the yellow colouration in off-white diamonds. Diamonds that are totally of either 1aA or 1aB type are colourless. All synthetic, but only a few natural diamonds are type 1b. The Tiffany diamond is a noted example of a natural fancy 1b.
• Boron is also of similar size to carbon and is often present in diamonds, but its presence is only noticeable when the nitrogen level is extremely low. The boron impurity, as an acceptor, transforms the lattice into a semi-conductor which also absorbs light in the red region of the spectrum. A blue colour results, for which the most famous example is the Hope diamond. Argyle blue or violet diamonds are not the result of boron impurities. Nickel or high concentrations of hydrogen are believed to be responsible for this colouration in these diamonds.
• The presence of colour in brown and pink diamonds remains a mystery. It has been established that lattice misalignments and ruptured bonds are present in all brown and pink diamonds. Levels of nitrogen are also important, with lower levels favouring pink colours. A significant feature of Argyle's pink diamonds is their chroma- and thermo-chromatic properties, by which the intensity of colour can be temporarily changed by either exposure to UV which bleaches the colour or exposure to heat which intensifies it. Such behaviour is indicative of charge transfers between two defect centres believed to be single nitrogen and another structure.
• Smaller scale lattice disturbances are generated by radioactive particles which dislodge carbon atoms from their positions. The resulting centre (GR1) gives rise to a green colouration. Because of the limited penetration of radioactive particles into a diamond, the green colour is usually confined to the outer layers, which will be mostly removed on polishing. Natural green diamonds are not widespread, the most notable example being the 'Dresden Green'.