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The hardest element is carbon, in the form of diamond. It scores 10 on the Mohs hardness scale — the maximum — and no other element comes close. That’s an odd result when you remember that pencil “lead” is also pure carbon, and it’s one of the softest solids you’ll handle.
How hardness is measured
Hardness means resistance to scratching or indentation. Two common scales:
- The Mohs scale (1812), from 1 to 10, ranks minerals by which can scratch which: talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, feldspar 6, quartz 7, topaz 8, corundum 9, diamond 10. It’s an ordering, not a proportional scale — the jump from 9 to 10 is far bigger than from 1 to 2.
- Vickers and Knoop tests press a diamond tip into the material and measure the dent, giving numerical values that can be compared directly. Diamond typically scores somewhere around 70–150 GPa on the Vickers scale depending on the crystal and direction; hardened steel is under 10.
Hardness isn’t the same as toughness. Diamond is extremely hard but can shatter if struck sharply along certain planes. Steel is much softer but far tougher.
Why diamond is so hard
In diamond, every carbon atom is bonded covalently to four other carbon atoms in a rigid three-dimensional network, each bond pointing to the corner of a tetrahedron. The C–C bonds are short and strong, and there are no weak directions: to scratch diamond, you’d have to break many strong covalent bonds.
Why graphite is soft
Carbon can also form graphite. There, each carbon bonds to only three neighbours, forming flat hexagonal sheets. The bonds within a sheet are actually slightly stronger than diamond’s, but the sheets themselves are held together only by weak forces. They slide over each other easily — which is why graphite works as a pencil and as a dry lubricant.
A single one of those sheets, graphene, is one of the strongest materials ever measured in tension. Same element, three completely different materials. These different forms of an element are called allotropes — see isotopes vs. ions vs. isomers vs. allotropes.
The next hardest elements
After diamond, the hardest elements are boron (Mohs roughly 9.3–9.5), which also forms a strongly bonded covalent network, and a few metals. Among metals, the hardest include:
| Metal | Approximate Mohs hardness |
|---|---|
| Chromium | ~8.5 |
| Tungsten | ~7.5 |
| Rhenium | ~7 |
| Osmium | ~7 |
| Iridium | ~6.5 |
Values for metals vary between sources because hardness depends heavily on purity, grain size and how the metal was processed.
Chromium is the hardest pure metal by most measures, which is one reason chrome plating is used to protect surfaces. Tungsten is extremely hard and dense; as tungsten carbide (a compound), it’s used for drill bits, cutting tools and armour-piercing ammunition.
The softest elements
At the other extreme, the alkali metals are so soft you can cut them with a butter knife: cesium and rubidium have Mohs hardness around 0.2–0.3, and sodium and potassium around 0.5. Among nonmetal solids, graphite and sulfur are soft too.
Harder than diamond?
Some materials are claimed to rival or exceed diamond in some measurements:
- Lonsdaleite, a rare hexagonal form of carbon found in some meteorite impact sites, has been predicted to be harder than diamond.
- Cubic boron nitride, a compound, is second only to diamond among common materials and handles heat better, which makes it valuable for machining steel.
Diamond remains the hardest naturally occurring bulk material you can reliably measure.
Quick answers
What is the hardest element? Carbon, as diamond.
What is the hardest metal? Chromium is usually named as the hardest pure metal.
Is tungsten harder than diamond? No. Tungsten carbide is very hard (about 9 on the Mohs scale), but diamond is harder.
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