Diamond and pencil lead are both pure carbon — so why are they so different in hardness?
― 150km underground, pressure repacked the atoms
Pencil lead crumbles into black smudges the moment it touches paper. Diamond is so hard that no other natural stone can scratch it. And yet the two are made of exactly the same atom: carbon. The difference lies in how those atoms are linked together — and that linkage was decided by an unimaginable pressure deep underground, in a place no human will ever reach.
You're writing in a notebook, and the side of your hand has turned black. Bit by bit, the pencil's graphite has been rubbing off onto the paper and your skin.
That same day, a diamond ring sparkles in a jeweller's display case. Wear it for decades, and it will barely wear down at all.
A black lead that stains your hand in seconds, and a clear stone praised for its eternal sparkle. It's hard to believe they're the same material.
Just two reasons explain the difference
In pencil lead (graphite), carbon atoms link up into flat sheets, and those sheets simply stack on top of one another. In diamond, every atom reaches out in four directions, locked into a rigid three-dimensional lattice.
For the same weight of carbon, diamond takes up nearly 40% less space. That tightly packed shape is only stable under the extreme pressures found deep underground.
The first point explains "why the hardness differs"; the second explains "why diamond is rare." Let's look at each in turn.
Stacked sheets versus a lattice reaching every which way
Look at the left side of Figure 1. In graphite, each carbon atom bonds strongly to three other atoms in the same flat plane, forming a honeycomb sheet of hexagons. Bonds within a sheet are very strong, but the sheets themselves are only loosely stacked, held together by weak forces.
So when you rub it on paper, the sheets slide apart and slough off onto the page. That's what a pencil line actually is. Graphite feels soft not because its atomic bonds are weak, but because it has an easy direction to peel apart along.
The right side of Figure 1 shows diamond. Here, each atom bonds to four others. Those four bonds spread out in every direction — up, down, left, right — weaving an unbroken three-dimensional lattice. Push from any direction, or try to peel any part away, and you must always break a strong bond. That's why it's the hardest natural substance known.
A compressed form only made deep underground
Compare the numbers at the bottom of Figure 1: for the same size, diamond weighs about 1.6 times more than graphite. In other words, its atoms are packed more tightly. Under strong pressure, matter tends to shift toward the shape with smaller volume. So under crushing pressure, diamond becomes the "settled shape" for carbon.
But that pressure is on another scale entirely. Most natural diamonds are thought to have formed beneath continents, at depths greater than roughly 150 kilometres (Figure 2). Temperatures there exceed 1000°C, and the pressure is about 50,000 times atmospheric pressure — equivalent to the weight of roughly 10 elephants pressing on an area the size of a fingertip.
At the weak pressures found at the surface, graphite is what settles for carbon instead. That's why graphite turns up commonly in surface rocks. Diamond, on the other hand, can only reach us if something carries crystals formed deep underground all the way up to the surface.
Once at the surface, why doesn't diamond turn back into graphite?
This raises a question. If graphite is the settled form at the surface, will diamond eventually revert back to graphite?
The answer is: "it wants to, but it can't." Rearranging a three-dimensional lattice means first breaking a great many strong bonds. At room temperature, there simply isn't enough energy available to trigger that. So the rate of change is thought to be far too slow to notice — not just in a human lifetime, but even across the whole of geological history.
If diamond rose slowly, it would risk turning into graphite while passing through hot zones along the way. Natural diamonds are thought to have survived because a special, gas-rich magma blasted them up near the surface in a matter of hours to days, as shown by the arrow in Figure 2. The rock left behind when that passage cools and solidifies is "kimberlite," the rock mined at diamond sites worldwide.
Diamond is pure carbon, so heating it in air well above 700°C makes it burn, just like charcoal, into carbon dioxide. In the late 18th century, the British chemist Smithson Tennant measured the gas produced by burning diamond and showed it was the same carbon found in charcoal. Unbeatable in hardness, but no match for heat and oxygen.
Dating tiny minerals trapped inside diamonds suggests many natural diamonds formed 1 to over 3 billion years ago. The eruption that carried them to the surface is far more recent, meaning they slept deep underground for an immense stretch of time after forming.
Summary
Pencil lead and diamond are both made of nothing but carbon. Graphite is just flat sheets stacked together, so it flakes apart easily; diamond is a three-dimensional lattice reaching in every direction, so it's extremely hard. And diamond's tightly packed shape only forms under the extreme conditions of about 50,000 times atmospheric pressure.
Same material, different arrangement, different substance entirely.
Diamond is a "record of pressure" that Earth's depths have etched into carbon.
Pressure changing the shape and properties of things comes up elsewhere too. Why crushing deep-sea pressure doesn't crush deep-sea creatures is covered in Why don't deep-sea creatures get crushed by that water pressure?, and why iron at Earth's core stays solid despite blistering heat is covered in Earth's core is as hot as the Sun's surface — so why isn't it molten?. For how atomic arrangement decides whether something floats or sinks, see Why does ice float on water? too.
- Using the darkest pencil you have (like a 6B), fill in a roughly 5cm square on paper solidly, pressing fairly hard, with no gaps.
- Tilt the shaded patch at an angle under a lamp. Even though it's black, it should flash silvery, almost like metal. Because the flat sheets all lie facing the same way, they reflect light like a mirror.
- Stick a piece of tape onto the shaded patch, rub it with your finger, then peel it off slowly. A thin grey film should transfer onto the tape, and the paper underneath will look slightly lighter. The easily-peeled sheets have stuck to the tape and come away.
Try comparing with 4B, 2B, and HB pencils too. Darker pencils contain a higher proportion of graphite, making the shine and transfer more obvious. This will get your hands dirty, so wash up with soap afterward.
Want to know more? ― Terms, formulas, and textbook connectionsClearly labelled by level, from middle-school science through university specialist courses
- MSCovered in middle-school science
- HSCovered in high-school Chemistry/Earth Science
- HS+High-school advanced content, or textbook sidebar material
- UnivNot covered in high school — university specialist content (chemical thermodynamics, petrology)
- ResearchNot yet settled even at university level — an active area researchers are still investigating
MSTerms: this phenomenon has a name
- Allotropes: substances made of the same kind of atom, differing in properties because the atoms are linked differently. Graphite and diamond are allotropes of carbon. Oxygen and ozone are another example.
- Covalent network crystal: a structure where atoms share electrons to bond, with that bonding extending unbroken throughout the whole crystal. Diamond is the classic example, giving it hardness and a high melting point.
- Kimberlite: rock formed when magma that rose rapidly from deep underground cools and solidifies. Known as the "elevator" that carries diamond to the surface.
MSHSCheck with a formula: how much pressure is there at 150km depth?
The weight of rock piled up above becomes the pressure pushing on the rock below. Pressure can be estimated as "rock density × gravitational strength × depth." Rock density underground actually varies with depth, but here we'll use an average estimate.
| Symbol | Meaning and unit |
| P | Pressure (pascals — force per square metre) |
| ρ | Rock density (kilograms per cubic metre) |
| g | Gravitational acceleration (metres per second squared) |
| h | Depth (metres) |
| Form of the equation | P = ρ × g × h |
| Average rock density underground (estimate) | about 3300 kg/m³ |
| Gravitational acceleration | 9.8 m/s² |
| Depth | 150 km (150000 m) |
| Atmospheric pressure | about 101300 Pa |
| Weight of one African elephant (estimate) | about 5000 kg |
| Multiply density and gravity | 3300 × 9.8 = 32340 |
| Multiply by depth for pressure (Pa) | 32340 × 150000 = 4851000000 |
| How many times atmospheric pressure | 4851000000 ÷ 101300 ≒ 47887 |
| Force per square centimetre (N) | 4851000000 ÷ 10000 = 485100 |
| Converted to weight (kg) | 485100 ÷ 9.8 = 49500 |
| How many elephants' worth | 49500 ÷ 5000 ≒ 9.9 |
The pressure at 150 kilometres depth is roughly 50,000 times atmospheric pressure — the same weight as about 10 elephants pressing on an area the size of a fingertip.
| Graphite density (estimate) | about 2.2 g/cm³ |
| Diamond density | about 3.5 g/cm³ |
| Volume ratio for the same weight | 2.2 ÷ 3.5 ≒ 0.63 |
| Fraction shrunk | 1 − 0.63 = 0.37 |
The same weight of carbon shrinks in volume by about 40% when it becomes diamond. Strong pressure pushes carbon toward this "smaller" shape.
HSHS+Why pressure swaps which shape is "settled"
HSIn chemistry, you learn that when conditions change, a system shifts in the direction that eases that change (Le Chatelier's principle). Raising the pressure shifts things toward diamond's smaller volume, easing the effect of the pressure.
HS+Graphite's carbon bonds in three directions within a plane, and its remaining electron can move freely across the whole sheet. That's why graphite conducts electricity and has a metallic sheen. Diamond's carbon uses all four electrons for bonding, so it barely conducts electricity and is transparent.
UnivPhase diagrams and "metastability"
A diagram showing which form has lower energy at each temperature and pressure is called a phase diagram. In carbon's phase diagram, diamond becomes the stable form even at room temperature once pressure exceeds roughly ten-thousand-odd atmospheres, and the boundary pressure rises with temperature. Surface diamond sits in a "metastable" state — higher in energy, but unable to change because the barrier to change (activation energy) is too high. Heating it to around 1500°C in the absence of oxygen is said to start converting it into graphite. Synthetic diamond was first made in the 1950s using this phase diagram as a guide, in high-temperature, high-pressure equipment.
ResearchWhat's still not fully understood
- How carbon crystallizes deep underground Carbon dissolved in underground fluids is thought to crystallize through oxidation-reduction reactions, but the exact nature of those fluids and where the reactions occur is still being studied.
- Diamonds from even greater depths So-called "super-deep diamonds," thought to form at depths of roughly 300–700 kilometres, have been found. Minerals trapped inside them offer clues that Earth's deep interior contains water, which has drawn a lot of attention.
- Why can magma rise so fast Gases such as carbon dioxide are thought to form bubbles that accelerate the ascent, but actual estimates of the speed vary widely.
In other words, this article too describes things "as best understood for now." Diamond is also one of the few "physical letters" that reach us straight from depths of the Earth people can never dig down to.
Textbook connections (by level)
| Level | Subject/unit | Where in this article |
|---|---|---|
| MS | Science: "Properties of matter," "Pressure," "How the Earth formed" | Density difference, pressure from the weight of rock, magma |
| HS | Chemistry: "Chemical bonding and crystals"; Earth Science: "Earth's interior" | Allotropes, covalent network crystals, underground temperature and pressure |
| HS+ | Advanced chemistry: "Le Chatelier's principle," "graphite's electrical conductivity" | How the settled form shifts with pressure, graphite's sheen |
| Univ | Chemical thermodynamics, petrology | Carbon's phase diagram, metastability, kimberlite |
| Research | Geochemistry, high-pressure mineralogy | Crystallization reactions, super-deep diamonds, magma ascent speed |
| ― | Everyday connections | Writing with a pencil, looking at diamond in jewellery or cutting tools |
- Shirey, S. B. et al. (2013) "Diamonds and the Geology of Mantle Carbon", Reviews in Mineralogy and Geochemistry 75
- Pearson, D. G. et al. (2014) "Hydrous mantle transition zone indicated by ringwoodite included within diamond", Nature 507
- Bundy, F. P. et al. (1955) "Man-made Diamonds", Nature 176
- National Astronomical Observatory of Japan (国立天文台), ed., Rika Nenpyō (理科年表, Chronological Scientific Tables), Maruzen Publishing
※This article is a general-audience science explainer. The figures given are estimates meant to aid understanding of the mechanisms involved. Estimates of underground temperature and pressure, and of diamond ages, vary across studies.