Diamond

The hardest known material at Mohs 10, and the scale is badly non-linear — diamond is several times harder than corundum at 9, not 11% harder. It has no melting point at ordinary pressure: heated in air it burns from about 700 °C, and in an inert atmosphere it converts to graphite rather than melting, because graphite is the thermodynamically stable form at 1 atm. Less famously, it conducts heat about five times better than copper, which is why it is used as a heat spreader under high-power laser diodes.

Properties

Density

3,515 kg/m³ — 3.52 g/cm³

Young's modulus

1050 GPa

Thermal conductivity

2000 W/(m·K)

Hardness

10 Mohs

Thermal expansion

1 µm/(m·K)

Floats in water

No

Magnetic

No

What it is used for

• Cutting, grinding and drilling tooling
• Gemstones
• Heat spreaders in high-power electronics
• Anvils for ultra-high-pressure research

How it compares

Does it rust?

Diamond contains no iron, so it cannot rust. Whether it degrades depends on the material, not on oxidation.

Is it magnetic?

Diamond is not a metal and has no ferromagnetic response.