Natural diamonds typically form 150–200 km below the surface of the earth. Diamond formation does not occur everywhere at these depths, but only below the oldest continents that have been stable for billions of years; these areas are known as cratons (see figure 2 in Summer 2018 Diamonds from the Deep).
The vast majority of diamonds form between about 150 to 200 km below Earth's surface. But a handful come from much deeper.
Diamond occurs naturally but is extremely rare compared to other minerals. Diamond is thought to form 150 km to 200 km below the Earth's surface, where high temperatures (1050°C - 1200°C) and pressures (45 kilobars - 55 kilobars) allow it to crystallise.
Diamond reserves have been stable in recent years at 2.3 billion carats, a sufficient level to sustain current annual production for the next 18 years. New pipeline and expansion projects along with currently operating facilities will generate around 172.3 million carats in 2025.
The top 5 diamond mining nations are Russia, Botswana, Democratic Republic of Congo (DRC), Australia and Canada. Just these five contribute to more than 60% of the world's diamond production.
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.
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.
Learn about five of the country's most unique gems. There are three famed diamond mines in Australia: Argyle, Ellendale and Merlin. Combined, the mines have supplied millions of carats worth of gems to the global market, and individually they have produced some of the rarest, most exquisite stones ever recovered.
The diamonds we find have been transported close to the earth's surface by special magmatic eruptions, forming the rocks known as kimberlite, and occasionally lamproite. This magma rises quickly from the mantle up to the surface, sometimes whisking diamonds along for the ride as accidental passengers.
In its elemental form, gold is significantly rarer than diamonds.
What may come as somewhat of a surprise though, is the fact that diamonds can be found beneath the ocean as well! Millions of years ago, the Orange River carried mass amounts of diamonds from the middle of South Africa and deposited these stones on the bottom of the sea.
Namibia is the country most commonly associated with marine diamonds. Its beaches were the scene of a brief rush in 1908, and the coastal area where they are found has long been known as the Sperrgebiet, which is name provided by the country's once German colonial masters, and literally means “Prohibited Area.”
Almost all commercially minable diamonds are formed in an area of the earth's mantle about 150 kilometers below the surface. This area is often known as the diamond stability zone, where temperatures are above 1000 degrees Celsius. There is also tremendous pressure at these depths, between 45- and 60-kilobars.
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.
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.
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.
Minerals are assigned a number between 1 and 10 on the Mohs Hardness Scale to describe how resistant to scratching they are. Diamonds are given the highest number, a 10. There is nothing that can scratch a diamond except another diamond.
Cleaving and sawing
Because diamonds are the hardest natural material on Earth, only other diamonds can cut them. Diamond-bladed edges are used on the equipment to make the cuts. In some cases, special lasers may be used to assist in this process.
Due to the immense pressure that is present in this part of the earth, as well as the extreme temperatures, a diamond gradually begins to form. The entire process takes between 1 billion and 3.3 billion years, which is approximately 25% to 75% of our earth's age.
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.
The Lynas Mt Weld mine in Western Australia is acknowledged as one of the world's premier rare earths deposits.
Leonard Joel has been busy building a reputation as the auction house of big-ticket jewels and million-dollar-plus diamonds. It holds the record for the most expensive diamond sold at auction in Australia – $1.625 million for a 20.05 carat rock, sold in December 2021.
Diamond-bearing kimberlite pipes are only found in ancient areas of the continents. But diamonds have also been found far from the pipes. This is because volcanic rock erodes and is washed away over time. Rivers, streams and glaciers can carry diamonds far from the original volcanoes.
Diamonds found at the Crater are typically smooth and well rounded. Their shape resembles a polished stone with smooth sides and rounded edges. Size: The average size of a diamond is about the size of a paper match head, approximately 20-25 points weight.