Diamonds are found in a surprisingly diverse range of rock types, but the overwhelming majority come from just two volcanic rocks: kimberlite and lamproite. These are the eruption pipes that carried diamonds from deep in the Earth’s mantle to the surface. The story is more layered than that, though, because the rocks where diamonds actually crystallize are different from the rocks that deliver them, and diamonds also turn up in sedimentary gravels, metamorphic terranes, and even meteorites.
Where Diamonds Actually Form
Before a diamond reaches the surface, it spends millions or even billions of years embedded in rock deep beneath the continents, typically at depths of 150 to 200 kilometers or more. The two main rock types where diamonds crystallize are peridotite and eclogite. Studies of mineral inclusions trapped inside diamonds show that roughly two-thirds of inclusion-bearing diamonds grew within peridotite, while about a third grew in eclogite.1Journal of Physics: Condensed Matter. Formation of diamond in the Earth’s mantle
Peridotite is the dominant rock of the upper mantle. It is rich in the minerals olivine and pyroxene and poor in silica compared to the rocks you walk on at the surface. Diamonds that grow within it tend to form in particularly depleted varieties like harzburgite or dunite, rocks that have already lost much of their melt-producing material. These are the leftovers of ancient melting events that date back billions of years, and diamond crystallization within them appears linked to fluids or melts filtering through the mantle and reacting with their surroundings.2Chemical Geology. Slab-mantle interactions: 2. The formation of diamonds
Eclogite, the second-most-common diamond host, has a very different backstory. It is made up primarily of garnet and a green pyroxene mineral called omphacite, and it represents ancient oceanic crust that was pushed deep into the mantle at subduction zones. When slabs of ocean floor plunge beneath a continent, some of that material gets stuck in the thick mantle root below, transforming into eclogite under extreme pressure. Carbon carried down with the slab can crystallize into diamond under the right conditions.3PubMed Central. Evidence for oxygen-conserving diamond formation in redox-buffered subducted oceanic crust sampled as eclogite Pieces of eclogite containing diamonds have been found carried to the surface as chunks entrained in kimberlite eruptions.4Geology. Caught in the act: Diamond growth and destruction in the continental lithosphere
The carbon that eventually becomes diamond can have several origins. In peridotitic settings, mantle-derived carbon-rich fluids percolate through the rock and deposit diamond as they cool or react chemically. In eclogitic settings, carbon isotope signatures sometimes point to a crustal origin, meaning organic carbon from sediments on the ancient sea floor was recycled deep into the Earth before becoming diamond.5Elsevier (Chemical Geology). Carbon isotopes of eclogite-hosted diamonds from the Nyurbinskaya kimberlite pipe, Yakutia: The metasomatic origin of diamonds In other words, some of the carbon in a diamond on your finger may once have been part of living organisms on an ancient ocean floor.
Kimberlite, the Classic Diamond Delivery System
Diamonds form deep in the mantle, but they can only become accessible if something brings them up fast enough to survive the journey. That something, in most cases, is kimberlite. Kimberlite is a volatile-rich volcanic rock loaded with water and carbon dioxide that erupts explosively from depths of over 150 kilometers. The eruption punches through the continental crust in a narrow, roughly carrot-shaped pipe called a diatreme. Most of the world’s major diamond mines, from the classic Kimberley pipes of South Africa to the large pipes of Siberia and Canada’s Northwest Territories, are kimberlite diatremes.
Speed matters enormously. Diamond is only the stable form of carbon under the enormous pressures found deep in the mantle. At the lower pressures near the surface, graphite is the stable form. A slow ascent would give the diamond time to convert to graphite, destroying it. Kimberlite magmas rise so rapidly that diamonds are essentially quenched, preserved before they can break down. The magma also carries up chunks of mantle and crustal rock, called xenoliths, which geologists study to understand conditions at depth.
Not every kimberlite contains diamonds. Thousands of kimberlite pipes are known worldwide, but only a fraction are diamondiferous, and fewer still are rich enough to mine profitably. Whether a particular pipe carries diamonds depends on whether its source region in the mantle had diamond-bearing rock, and whether the eruption was fast and cool enough to preserve the crystals during ascent.
Lamproite and Orangeite
Kimberlite gets most of the attention, but it is not the only volcanic rock that brings diamonds to the surface. Lamproite, a potassium-rich volcanic rock, was thrust into the spotlight in the 1980s when the Argyle deposit in Western Australia turned out to be the world’s largest diamond source by volume. Argyle produced more than 750 million carats over its lifetime before closing in 2020.6Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits. The Discovery of the Argyle Pipe, Western Australia: The World’s First Lamproite-Hosted Diamond Mine The deposit formed when a lamproite magma erupted through an ancient rift zone, sampling diamonds from the thick mantle root beneath the Kimberley craton.7PubMed Central. Emplacement of the Argyle diamond deposit into an ancient rift zone triggered by supercontinent breakup
The Argyle pipe’s diamonds had a mixed heritage. The mantle root beneath it contained both older peridotitic diamonds, likely of Archean age, and younger eclogitic diamonds with distinctive light carbon isotope signatures, pointing to subducted crustal carbon.8Geoscience and Exploration of the Argyle, Bunder, Diavik, and Murowa Diamond Deposits. Nature of the Mantle Beneath the Argyle AK1 Lamproite Pipe: Constraints from Mantle Xenoliths, Diamonds, and Lamproite Geochemistry That combination of old and young diamonds contributed to Argyle’s extraordinary richness, though many of its stones were small and brownish pink rather than the colorless gems typically associated with kimberlite mines.
Orangeite, historically called Group II kimberlite, is another diamond host. These are potassium-rich, water- and carbon-dioxide-rich volcanic rocks found mainly in southern Africa. Like kimberlite, they erupt through diatremes and carry fragments of mantle rock to the surface.9PubMed Central. Did diamond-bearing orangeites originate from MARID-veined peridotites in the lithospheric mantle? Orangeites are less well known than kimberlites, partly because fewer economic deposits have been found in them, but they represent an important variation in the volcanic plumbing that can deliver diamonds.
Lamprophyres and Other Rare Volcanic Hosts
Beyond kimberlite, lamproite, and orangeite, a handful of other volcanic rock types have yielded diamonds, though rarely in economic quantities. Lamprophyres are a family of dark, volatile-rich igneous rocks that sometimes carry diamond. They come in several subtypes: some are linked to subduction zones, where water released from descending slabs helps create their unusual chemistry, while others erupt along continental rifts from deep mantle sources. Diamondiferous lamprophyres are considered the rarest of all diamond-bearing rock types. Part of the reason is that lamprophyre melts tend to be chemically hostile to diamond, with high oxygen levels and abundant water and carbon dioxide that can corrode or destroy crystals before they reach the surface.10Earth-Science Reviews. Occurrence and petrogenesis of diamondiferous lamprophyres: Insights into mantle metasomatism and diamond formation
These rare hosts are more important scientifically than commercially. Each one tells geologists something about the range of volcanic processes that can, under the right circumstances, sample the diamond-bearing parts of the mantle and carry crystals upward. Finding diamonds in an unexpected rock type can reshape understanding of the geology below a particular region.
Sedimentary Deposits and Alluvial Diamonds
A large share of the world’s diamonds, historically speaking, have been found not in the volcanic pipes where they first arrived at the surface, but in sedimentary deposits formed by weathering, rivers, and ocean currents. Once a kimberlite or lamproite pipe erodes over millions of years, its diamonds are liberated and carried downstream. Because diamond is among the hardest and most chemically resistant minerals, it survives the journey that destroys most other minerals. The diamonds concentrate in gravel beds, riverbeds, and coastal deposits called placers.
The Atlantic coast of southwestern Namibia is one of the most famous examples. There, Pliocene to Holocene shoreline gravels have yielded more than 63 million carats of gem-quality diamonds. The richest deposits are fossil beach gravels preserved over hard bedrock, where diamonds settled into traps along ancient coastlines.11Earth Surface Processes and Landforms. Diamondiferous gravel barrier beach placer: Morpho‐sedimentary dynamics and diamond distribution connectivity, SW Namibia Mining these deposits involves stripping away sand to expose the bedrock and collecting diamonds from the gravel trapped in its crevices, sometimes even from the seabed using specialized dredging ships.
Alluvial diamonds were, in fact, the first diamonds ever found by humans. The famous diamond fields of India, Brazil, and the early South African river diggings were all placer deposits. It was only later that geologists traced these alluvial stones back to their kimberlite sources. Today, alluvial and marine deposits remain economically significant in several African countries and parts of South America, though they are increasingly difficult to mine as the easier gravels are exhausted.
Diamonds in Metamorphic Rocks
Diamonds also form in a completely different geological setting: ultrahigh-pressure metamorphic terranes. When two continental plates collide, the leading edge of one plate can be shoved to depths where pressures are great enough to form diamond. These are not the large, gem-quality stones you see in jewelry. They are microdiamonds, typically microscopic crystals embedded inside metamorphic minerals like garnet. Their significance lies not in commercial value but in what they reveal about how deeply continental crust can be buried and then brought back to the surface.
Microdiamonds of this type are important markers of deep subduction. Their presence in a rock proves that the rock experienced pressures normally found only at mantle depths, even though it is now exposed at the surface.12PubMed Central. Metamorphic microdiamond formation is controlled by water activity, phase transitions and temperature Several well-studied localities exist around the world. In Norway’s Arctic Caledonides, for example, researchers have documented microdiamonds sitting inside fluid inclusions within garnet crystals in metamorphic gneiss, confirming that these rocks were once at extreme depths before being exhumed to the surface.13Journal of Petrology. Metamorphism and Partial Melting at UHP Conditions Revealed by Microdiamonds and Melt Inclusions in Metapelitic Gneiss from Heia, Arctic Caledonides, Norway
Other UHP localities where metamorphic diamonds have been found include the Kokchetav Massif in Kazakhstan and the Dabie-Sulu belt in China. In each case, the diamonds are tiny, often a few tens of micrometers across, and their host rocks are gneisses, schists, or other metamorphic types that were originally continental crustal material. You will never see these diamonds in a ring, but they are among the most powerful tools geologists have for reconstructing the collision history of continents.
Diamonds From Space and Impact Events
Diamonds are not exclusive to Earth. One of the more unexpected places they turn up is inside meteorites, particularly a class called ureilites. Most ureilite meteorites contain diamonds, and they are generally thought to have formed when high-speed impacts converted carbon-rich material to diamond through shock compression.14Geophysical Research Letters. Raman spectroscopy of diamond in ureilite and implication for the origin of diamond These diamonds are nanometric, with crystal sizes often around 20 to 25 nanometers, far too small to see without electron microscopy.15PubMed Central. Impact shock origin of diamonds in ureilite meteorites
The origin of ureilite diamonds has been debated for decades. The leading explanation is shock conversion during the catastrophic breakup of the ureilite parent body in the early solar system, within the first ten million years after the solar system formed.16PubMed Central. A large planetary body inferred from diamond inclusions in a ureilite meteorite Some researchers have argued that larger diamonds found in certain ureilite samples point to sustained high pressures inside a Mercury-to-Mars-sized planetary body, rather than brief shock events. That would mean the diamonds grew slowly deep inside a now-destroyed protoplanet, which is a remarkable thought: wearing a ureilite diamond (if one could be large enough) would mean wearing a piece of a dead world.
On Earth, impact events can also produce diamonds. When a large asteroid or comet slams into graphite-bearing target rocks, the shock wave generates pressures and temperatures high enough to convert graphite to diamond in an instant. The Popigai crater in Siberia, formed about 36 million years ago, is the best-known example. The impact struck graphite-rich gneiss, and the resulting diamonds are mixed with a rare hexagonal form of diamond called lonsdaleite. These impact diamonds are generally of industrial rather than gem quality, but the Popigai deposit is so enormous that it dwarfs conventional diamond reserves by sheer volume, even though there has never been a commercial case for mining them.
Why Cratons Matter
One pattern jumps out when you map the world’s major diamond deposits: almost all of them sit on cratons, the ancient, stable cores of continents. Cratons are the oldest parts of the continental crust, typically formed more than a billion years ago, and they are underlain by unusually thick mantle roots extending 150 kilometers or more beneath the surface.17ScienceDirect. The thermal evolution of cratonic roots from assembly to maturity: Constraints from Archean diamonds and modeling of peridotite melting That thick, cool root is the key. It provides the sustained high pressures needed for diamond to remain stable over geological time, and it stays cool enough that diamonds stored within it do not convert to graphite.
Younger, thinner parts of the continental crust simply do not have deep enough roots to reach the diamond stability field. This is why you find diamond mines in the ancient hearts of southern Africa, Siberia, Canada, Australia, and Brazil, but not in geologically young mountain belts or along active plate boundaries. The exceptions that exist, like certain deposits at the margins of cratons, still rely on proximity to ancient thick mantle. Argyle, for instance, sits at the edge of the Kimberley craton, and its diamonds were sampled from the craton’s deep root even though the pipe itself is at the margin.7PubMed Central. Emplacement of the Argyle diamond deposit into an ancient rift zone triggered by supercontinent breakup
For diamond explorers, this cratonic association is the first filter. Before investing in prospecting, companies look for regions underlain by old, thick lithosphere. Then they look for indicator minerals in stream sediments: garnets, chromites, and other minerals that erode from kimberlite or lamproite pipes. If the indicator chemistry suggests the right deep-mantle conditions, it becomes worth searching for the volcanic pipe itself. The vast majority of kimberlite pipes discovered through this process turn out to be barren or sub-economic, but the ones that do contain diamonds can be spectacularly valuable.
Why Some Rocks Destroy Diamonds Instead of Preserving Them
A natural question is why diamonds survive in some volcanic eruptions but not others. The answer comes down to the chemistry and speed of the magma. Kimberlite and lamproite melts rise from great depth and erupt explosively, giving diamonds little time to react with their surroundings. They also tend to have relatively low oxygen content during the initial stages of ascent, which helps preserve diamond. By contrast, some magma types are much more oxidizing or ascend more slowly. Lamprophyre melts, for instance, tend to carry high levels of dissolved water, carbon dioxide, and oxygen, and they erupt in smaller volumes with longer residence times. This combination creates an environment that actively corrodes diamond during transit.10Earth-Science Reviews. Occurrence and petrogenesis of diamondiferous lamprophyres: Insights into mantle metasomatism and diamond formation
Even within kimberlite systems, diamonds can be partially destroyed. Some kimberlite pipes contain diamonds with distinctive etching and resorption features on their surfaces, evidence that the crystals were being eaten away by the surrounding magma during ascent. The balance between preservation and destruction is one reason why diamond grade varies so dramatically from one pipe to the next. A kimberlite that took a slightly slower, hotter, or more oxidizing path to the surface might have started with the same diamond load as its neighbor but arrived with far fewer surviving crystals.
This also helps explain why mantle xenoliths, the chunks of deep rock carried up inside kimberlite pipes, are so scientifically valuable. A xenolith that still contains diamonds in place gives geologists a frozen snapshot of the mantle conditions where the diamonds grew and the environment they endured during eruption. Studies of these xenoliths have shown that diamonds can grow and be destroyed in the same mantle region over geological time, with multiple episodes of fluid infiltration building new diamond on the remains of old, partially corroded crystals.4Geology. Caught in the act: Diamond growth and destruction in the continental lithosphere
Diamonds You Cannot See
When most people think of diamonds in rocks, they picture visible crystals. But many diamond-bearing rocks contain crystals too small to see without specialized equipment. Metamorphic microdiamonds, as discussed above, are typically measured in tens of micrometers. Meteoritic nanodiamonds are smaller still, with crystal sizes often around 20 nanometers, roughly a thousand times thinner than a human hair.15PubMed Central. Impact shock origin of diamonds in ureilite meteorites Even some kimberlites contain populations of microdiamonds that are invisible to the naked eye but become apparent in laboratory analysis of processed rock samples.
Microdiamond sampling is, in fact, a standard exploration tool. When geologists evaluate a new kimberlite discovery, they often begin by processing small bulk samples and examining them under magnification for microdiamonds. The abundance and character of these tiny crystals can predict whether larger, commercially valuable stones are present in the pipe. A kimberlite with abundant microdiamonds of a certain size distribution is a more promising target for a larger, more expensive bulk sampling program. Conversely, a pipe with very few microdiamonds is unlikely to yield much at any scale.
This invisible diamond population also has implications for how much diamond exists on Earth overall. The total amount of diamond in the mantle is far larger than what has ever been mined. Much of it is in microcrystals dispersed through peridotite and eclogite hundreds of kilometers underground, never destined to reach the surface. Studies of seismic wave velocities have even suggested that certain deep mantle layers may contain trace amounts of diamond as a broadly distributed mineral phase. The diamonds we find in rings and industrial tools represent a tiny, biased sample of what the planet actually contains, filtered by the rare volcanic events violent enough to deliver them intact.