Most natural diamonds formed about 150 to 200 km below the surface, in the mantle beneath the oldest parts of the continents. Volcanic eruptions of unusual magma, mostly kimberlite, carried them to the surface in narrow pipes. Erosion later freed some diamonds from those pipes and washed them into rivers, beaches and the seabed. Most diamonds are between about 1 and 3.5 billion years old, far older than the rocks that brought them up.
Where diamonds form
Diamond is a high-pressure form of carbon. It is stable only where pressure and temperature are both high, which on Earth means deep in the mantle. The diamond chemistry and formation page explains the carbon structure. Here the question is where those conditions exist.
The diamond stability field begins at roughly 4.5 to 6 gigapascals of pressure, or 45,000 to 60,000 times atmospheric pressure, with temperatures around 900 to 1,300 degrees Celsius. Under most of the planet, the mantle at that depth is too hot. Under cratons, it is cool enough.
Cratons and their deep roots
Cratons are the ancient, stable centers of continents, mostly more than 2.5 billion years old. They have thick, cold roots of mantle rock, called the lithospheric mantle, that extend down 200 km or more. Inside these roots, a zone between about 150 and 200 km deep sits in the right pressure and temperature range for diamond to grow and survive for billions of years.
This explains why the world's diamond deposits in the host rock cluster on old cratons: the Kaapvaal craton in southern Africa, the Slave craton in northern Canada, the Siberian craton in Russia and others. Geologists summarize the pattern as Clifford's rule, after the geologist Tom Clifford, who noted in the 1960s that diamond-rich kimberlites occur on the oldest cratons.
How diamonds grow
Diamonds crystallize from carbon-bearing fluids or melts moving through the mantle rock. Two main families of host rock produce them:
- Peridotitic diamonds grew in peridotite, the dominant rock of the upper mantle.
- Eclogitic diamonds grew in eclogite, a dense rock thought to derive in many cases from ancient ocean crust pushed down into the mantle.
The minerals trapped inside diamonds tell geologists which family a stone came from. Small inclusions of garnet, olivine, pyroxene or sulfide are pieces of the mantle sealed in diamond.
A small number of diamonds come from much deeper, in the range of about 300 to 800 km. These superdeep diamonds include many large Type IIa stones and some blue Type IIb diamonds. Their inclusions preserve minerals that only form at those depths.
Kimberlite pipes
Diamonds would stay in the mantle if not for rare eruptions. Kimberlite is a volatile-rich, magnesium-rich igneous rock that forms deep in the mantle and rises fast, picking up chunks of mantle rock and any diamonds in them along the way. The rock takes its name from Kimberley, South Africa, where it was first studied after the diamond rushes of the 1870s.
Kimberlite magma travels to the surface quickly, likely in hours to days, and erupts explosively as dissolved gases expand. This speed matters. Diamonds left at high temperature and lower pressure for long would turn to graphite or burn, and a slow ascent would destroy them.
The eruptions leave carrot-shaped pipes that narrow with depth. Near the surface the pipe is filled with broken, mixed rock; deeper down it passes into feeder dikes. The famous Big Hole at Kimberley was dug into one of these pipes. Kimberlite eruptions vary widely in age, from more than a billion years ago to tens of millions of years ago, and the diamonds they carry are usually far older than the eruption itself.
Most kimberlites contain few or no diamonds, and only a small fraction hold enough gem-quality stones to mine. The rock cycle and igneous rocks page places kimberlite among other igneous rocks.
Lamproite pipes
Lamproite is a different potassium-rich volcanic rock that can also carry diamonds. The best-known example was the Argyle mine in Western Australia, a lamproite pipe famous for brown and pink diamonds, which closed in 2020. Lamproite pipes tend to be shallower and wider than kimberlite pipes, with a champagne-glass shape. The Ellendale field in Western Australia is another lamproite source. The diamond mines of Australia page covers both.
Indicator minerals
Diamonds are rare even in good kimberlite, so explorers look for more common minerals that come from the same mantle source. These indicator minerals include:
- Pyrope garnet, especially purple, chromium-rich, low-calcium types classed as G10 garnets, which match the garnets found inside diamonds.
- Chrome diopside, a bright green pyroxene.
- Picroilmenite, a magnesium-rich ilmenite.
- Chromite, a chromium oxide.
Indicator minerals survive weathering and spread out from a pipe in streams or glacial deposits. By sampling soil, stream sediment or glacial till and tracing the trail of indicators back to its source, geologists can locate a hidden pipe. That method led to the 1991 discovery of diamond-bearing kimberlites at Lac de Gras in Canada's Northwest Territories, described on the diamond mines of Canada page.
Alluvial and placer deposits
Once a kimberlite pipe reaches the surface, erosion starts wearing it down. Diamonds are hard and chemically resistant, so they survive while the softer host rock breaks apart. Rivers carry them downstream and drop them in gravel bars, terraces and old channels, where they concentrate with other heavy minerals.
These alluvial or placer deposits supplied the world's diamonds for most of history. India's Golconda region, Brazil's Minas Gerais and the river systems of West and Central Africa are all alluvial sources. Alluvial diamonds tend to be of high gem quality, because flawed and fractured stones break up during transport while sound crystals survive.
Some alluvial diamonds have traveled hundreds of kilometers from their source, and in places the original pipe has never been found.
Marine deposits
The Orange River in southern Africa carried diamonds from inland kimberlites to the Atlantic coast for millions of years. Waves and currents spread them along beaches and onto the continental shelf of Namibia and South Africa. These marine and beach deposits contain a very high proportion of gem-quality stones. The diamond mines of Namibia page covers the coastal and offshore deposits and how they are mined.
Other natural diamonds
Tiny diamonds also form in a few unusual settings. Meteorite impacts can create small diamonds from carbon in the target rock, and some meteorites contain nanodiamonds. Microdiamonds also occur in some metamorphic rocks that were pushed to great depth and returned. None of these produce gem material.
For how diamonds are extracted from pipes and gravels, see how diamonds are mined, and for the major deposits by continent, see the diamond mines of the world overview.
Frequently asked questions
How deep do diamonds form?
Most form about 150 to 200 km below the surface in the mantle beneath ancient cratons. A small number of superdeep diamonds come from about 300 to 800 km down.
Do diamonds form from coal?
No. Coal forms near the surface from plant material, while diamonds grow in the mantle far below any coal seam. Most diamonds are also older than land plants.
How old are diamonds?
Most natural diamonds are between about 1 and 3.5 billion years old, based on dating their mineral inclusions.
What is a kimberlite pipe?
It is a carrot-shaped body of volcanic rock left by a fast eruption from deep in the mantle. Some kimberlite pipes carry diamonds to the surface.