Diamonds are known to be carried to the earth's surface in only three rare types of magmas: kimberlite, lamproite, and lamprophyre. Of the three types, kimberlites are by far the most important, with several hundred diamondiferous kimberlites known.
Diamond is only formed at high pressures. It is found in kimberlite, an ultrabasic volcanic rock formed very deep in the Earth's crust. The extreme pressures needed to form diamonds are only reached at depths greater than 150km.
Kimberlite, along with a similar rock called lamproite, is important for delivering diamonds to the crust through magmatic intrusions that solidify into pipelike structures. Kimberlite occurs in the uplifted centres of continental platforms.
Surprisingly, most gemstones are found in sedimentary deposits after they have been weathered out of their original parent rock. These gem materials, discovered in river sediments and known as alluvial deposits, include gems such as diamond, spinel, sapphire and ruby.
The world's diamond deposits are largely found in Africa, Russia, Australia and Canada.
The only hardness test that will identify a diamond is scratching corundum. Corundum, which includes all rubys and sapphires, is 9 on the hardiness scale. If your suspected diamond crystal can scratch corundum, then there is a good chance that you found a diamond. But NO OTHER HARDNESS TEST will identify a diamond.
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.
A grove of Pandanus candelabrum, which appears to grow only in diamond-bearing kimberlite soils. Diamonds are formed hundreds of kilometers below the surface, as carbon is squeezed under intense temperatures and pressures.
Indicator minerals for diamond include, in order of decreasing significance: garnet, chromite, ilmenite, clinopyroxene, olivine, and zircon. But the order of persistence in streams is zircon, ilmenite, chromite, garnet, chromian diopside, and olivine. Diamond itself is obviously a most important indicator.
Diamonds are a type of metamorphic rock. Metamorphic rock is created when sedimentary or igneous rock changes as a result of high pressure and heat acting on the rocks. Metamorphic rocks have the characteristic of being hard and containing crystals. Diamonds are the hardest material on Earth.
The vast majority of diamonds form between about 150 to 200 km below Earth's surface. But a handful come from much deeper.
Lamprophyric (including kimberlitic) rocks are widespread in South Australia.
Diamonds are often found in the gravel layer, which collects under layers of other material, such as mud, clay and underwater plant-life. Once the gravel is collected, it is hauled to the surface and prepared for processing.
The most common separation method is called dense media separation or DMS. A DMS plant also uses the principal that diamonds are heavier than most of the surrounding rocks and minerals. Most modern DMS plants utilize a hydrocyclone (or 'cyclone'), which is essentially a large centrifuge.
Perform a scratch test. Scrape your gem against corundum and if it makes a scratch, you've got a real diamond on your hands. Diamonds have a density of 3.5 - 3.53 g/cm3, which you can test by dividing the weight of the stone by the weight of the stone suspended in water. Or, take your gem into a professional.
On the other hand, raw diamonds are uncut and unpolished. In other words, they have not been altered or tampered with after they were discovered. Raw diamonds look like transparent stones with yellowish or brownish tints. There are some that are colorless but these are rare.
Bristol Diamonds are quartz crystals found in geodes and geological rock formations which occur in dolomitic conglomerate in the Avon Gorge in Bristol, England. Their origin lies in geological processes of the Triassic period, about 250 to 200 million years ago.
Diamond occurs as a detrital mineral but its concentration in sand is always very low. However, diamond-bearing kimberlite pipes are often found by working through many fluvial sand samples but it is not diamond that prospectors are looking for.
Alluvial diamond mining occurs in riverbeds and beaches, where thousands of years of erosion and natural forces such as wind, rain, and water currents wash diamonds from their primary deposits in kimberlite pipes to beaches and riverbeds. Some alluvial deposits are from long-ago rivers.
The Kimberley region, in northern Western Australia, has been a prolific source of diamonds since the 1980s. Western Australia is particularly famous for its production of coloured diamonds and has become the primary source of pink diamonds for the world's markets.
Argyle diamonds are an extremely rare type of diamond. They are so rare that they have a market value of over $100,000 per carat. Argyle diamonds are the most expensive type of diamond found in nature. In 1868, they were discovered in the Argyle mine in Kimberley, Australia, and have been mined there ever since.
Black Diamond Coal Mine (Black Diamond A; Black Diamond B), Collie Shire, Western Australia, Australia.
Raw diamonds are not totally transparent but are more translucent compared to polished diamonds. You can see into a raw diamond but you cannot see through it. In raw form, they usually look like pale colored glass with hints of yellow or brown hues.
When mined from the earth, diamonds look like cloudy rocks before they're cut and polished. Their chemical nature and structure were unknown for centuries. It was Isaac Newton's experiments in the 1600s that first suggested diamonds are made up of the fourth-most abundant element, carbon.