In short, here is how diamonds are formed: carbon crystallizes roughly 150 to 200 kilometers down in the Earth's mantle, at about 900°C to 1,300°C, and under pressure tens of thousands of times greater than at sea level. Volcanic eruptions through kimberlite pipes later carry the crystals to the surface, often within hours to days.
The process that makes a diamond is more extreme and older than most people realize. Many natural diamonds are billions of years old, and the oldest dated stones formed 3.5 to 3.3 billion years ago, roughly three-quarters of the Earth's age.
That timescale is why a full answer to how diamonds are formed takes more than one sentence.
This guide covers three aspects of how diamonds form naturally: where they form, how a stone reaches the surface, and why the coal myth does not hold up.\
And one myth we’ll clear up is that Diamonds do not form from coal, and most are far older than the land plants from which coal is made from.
For a raw diamond, this story is the whole point of why it’s such a prized possession. A cut gem is shaped by a cutter. Guardian Diamond leaves the stone as that process made it.
Key Takeaways
- Most diamonds form under extreme pressure and temperature, roughly 150 to 200 kilometers below the Earth's surface.
- Many natural diamonds are billions of years old, and the oldest dated stones are 3.3 to 3.5 billion years old, far older than any land plant.
- Diamonds are not formed from coal. Coal forms from plant debris near the surface, while diamond-forming conditions exist much deeper and, for most diamonds, predate land plants entirely.
- Kimberlite eruptions are the primary known mechanism that carries diamonds from the mantle to the surface quickly enough for them to survive.
- Diamond's tightly bonded carbon lattice makes it the hardest known natural material, with a hardness of 10 on the Mohs scale.
Where and How Diamonds Actually Form
How diamonds are formed underground depends on a narrow zone of the earth's mantle, roughly 150 to 200 kilometers below the surface, where pressure and temperature are high enough to force carbon atoms into diamond's tightly bonded crystal structure.
Most diamonds crystallize at the base of cratons, the old and thick cores of continents. The temperatures there run from 900°C to 1,300°C.
Graphite at the Surface, Diamond at Depth
At the Earth's surface, graphite is the stable form of carbon, with a density of 2.26 g/cm³. Diamond packs the same atoms into a structure 56% denser, at 3.52 g/cm³. Only great confining pressure keeps that denser arrangement stable, which is why diamond forms in the mantle. That pressure is measured in gigapascals, and one gigapascal is about 10,000 times atmospheric pressure at sea level.
Crystal Growth in the Mantle
Under these conditions, each carbon atom bonds to four neighbors in a repeating tetrahedral lattice.
That rigid structure gives a raw diamond its hardness and, in clean crystals, its clarity.
Most growth is metasomatic, meaning carbon-bearing fluids, mixtures of water, methane, and carbon dioxide, move through solid mantle rock. As temperature and pressure shift, the carbon crystallizes out as diamond. The crystal builds layer by layer over thousands, or perhaps millions, of years, and may then rest in the mantle for billions more.
The Source of the Carbon
The carbon comes from two broad sources: carbon already present in the mantle, and carbon carried down by subducting oceanic plates in sediments and altered oceanic crust.
So some diamonds contain material that was once near the Earth's surface. The oldest dated diamonds, from the Diavik and Ekati mines in northern Canada, formed 3.5 to 3.3 billion years ago. The Earth itself is 4.54 billion years old. Not every diamond is that ancient, and some measured at Jagersfontein and Koffiefontein in South Africa are only about 90 million years old.
Evidence Inside the Stone
A raw diamond preserves evidence of how diamonds form in nature. Mineral grains and fluids sealed inside at depth are what scientists use to measure a stone's age, and the stone's natural shape, color, and inclusions reflect the same history.
Our octahedron diamond guide shows how one common crystal form reflects this growth, and our colored rough diamonds guide explains how color records it.
How Do Diamonds Reach the Surface?
A diamond that formed 150 kilometers underground only becomes findable because of a specific, violent kind of volcanic eruption that brings it up fast enough to survive the trip. Once you know how diamonds are formed, the eruption accounts for how one ever reaches the surface.
Kimberlite and Lamproite Pipes
Kimberlites, and to a lesser extent lamproites, are the most deeply derived magmas to erupt at the Earth's surface.
They are thought to begin as low-degree partial melts in the asthenosphere, producing a carbonate-rich magma. Water and carbon dioxide dissolved in that magma help crack the overlying rock and push the melt upward. Along the way, the magma tears loose pieces of rock and crystal, occasionally including diamonds.
The Ascent
The journey covers roughly 200 to 300 kilometers, and the magma can reach the surface in a matter of hours to days. Speed protects the stone, since the change in pressure and temperature threatens to oxidize or graphitize diamonds on the way up.
A slow ascent would give that change more time to work. Once a diamond reaches the surface, its hardness, strength, and chemical robustness let it survive there indefinitely.
How Few Pipes Hold Diamonds
There are about 7,000 known kimberlites and related mantle-sourced rocks worldwide. Only about 1,000 of them are of potential economic interest for diamonds. Fewer than 100 productive diamond mines have existed, past or present.
At most of them, diamonds are so sparse that they become visible only at the final stages of ore processing, and a decent ore grade is around one carat per tonne of rock.
After the Eruption
By the time a raw diamond reaches a jeweler, it has already survived formation and the eruption. We set it without cutting or polishing it.
Do Diamonds Really Form From Coal?
How diamonds are formed from coal is a question with a short answer: they are not, and for most natural diamonds, the timing alone makes it impossible.
The similarities are that they are both mined from the ground, but using very different methods. They are also both formed from carbon. Coal gets its carbon from prehistoric plants. Diamonds got their carbon from trapped primordial gases inside the Earth when the planet was first forming.
Depth and Setting
Coal is a sedimentary rock formed from plant debris deposited at the Earth's surface. It is rarely buried deeper than about 3.2 kilometers. Diamond-forming conditions sit 150 to 200 kilometers down.
Coal seams are also usually horizontal or nearly so, while the rock that delivers diamonds to the surface is a vertical pipe of igneous rock.
Age
The earliest land plants appeared about 450 million years ago. The oldest dated diamonds formed 3.5 to 3.3 billion years ago, and many predate the Cambrian explosion 541 million years ago.
Coal requires plant material, so most diamonds had already formed before any coal could exist. "Most" leaves room for exceptions: a few measured diamonds are only about 90 million years old, though they too grew in the mantle.
In short, coal forms near the surface from plant debris, while most diamonds formed deep in the mantle before land plants existed.
Natural Formation vs. Lab-Grown Diamonds
Lab-grown diamonds are made of the same material as natural diamonds, but they form in a matter of weeks under controlled laboratory conditions rather than over a geological timescale underground.
HPHT and CVD Growth
Two methods dominate lab growth. High-pressure, high-temperature (HPHT) growth mimics some of the conditions in which natural diamonds form. Chemical vapor deposition (CVD) builds a crystal layer by layer at near-vacuum pressure, a process very different from natural formation (GIA). Both begin with a diamond seed or substrate that supplies the crystal blueprint.
Comparing the Two Stones
Chemically and structurally, a lab-grown diamond and a natural diamond are both diamonds. They essentially have the same chemical composition and crystal structure, so the difference is origin and timescale, not composition.
Time does leave a trace, though. During billions of years in the mantle, nitrogen atoms slowly rearrange into new configurations, a geologic signature that very brief laboratory growth cannot produce (GIA, 2024).
Put simply, a lab-grown stone and a natural one share the same composition and structure. They differ in how long they took and where they formed.
Guardian Diamond's Stones
Guardian Diamond works with natural, raw, uncut diamonds, stones formed by the process described above, and left in the state that process gave them. Our ethical diamond sourcing guide explains how we approach sourcing. Whichever origin a reader prefers, knowing how each stone formed gives the label a concrete meaning.
The Four Stages of Diamond Formation
Carbon crystallizes under extreme pressure and heat deep in the mantle, the crystal grows over thousands to millions of years and may then wait for billions. A kimberlite eruption carries it up within hours to days, and only then can anyone find it.
A raw diamond carries this entire process in visible form. Guardian Diamond's pieces are chosen specifically to keep that record intact. For the fuller picture of our brand, start with what a Guardian Diamond is.
FAQs
How long does it take for a diamond to form?
There is no single figure. Crystal growth can take thousands or perhaps millions of years, and a stone may then rest in the mantle for billions more before an eruption carries it up. The oldest dated diamonds formed 3.5 to 3.3 billion years ago.
Are diamonds still forming today?
Possibly. The deep-earth conditions that produce diamonds still exist, and GIA notes that diamonds may be forming now and could be carried up by a kimberlite eruption millions of years from now. The only diamonds available to people are the ones already at the surface.
Why are diamonds found in certain places and not others?
Diamonds reach the surface through kimberlite and lamproite pipes, and those eruptions are rare. There are about 7,000 known kimberlites and related rocks worldwide, and only about 1,000 are of potential economic interest. River and coastal deposits form where weathering has carried diamonds away from their original pipes.
Is it true diamonds come from coal?
No. Coal forms near the surface from plant debris, while most diamonds formed deep in the mantle before land plants existed.
What's the difference between how natural and lab-grown diamonds form?
Both are diamonds, with the same composition and structure. The difference is time and place. Natural diamonds form over geological time in the mantle, while lab-grown diamonds are grown in a matter of weeks using HPHT or CVD methods.
Leaving the Stone Uncut
A diamond's formation spans billions of years and hundreds of kilometers of rock, and Guardian Diamond leaves its stones uncut so they stay as the earth made them.
Explore Guardian Diamond's talisman pieces and find the one that speaks to your energy.
