The Mohs hardness of boron carbide (B₄C) is approximately 9.3 to 9.5.
To put that in context, here is its position on the Mohs scale:
9: Corundum (ruby, sapphire)
9.3–9.5: Boron Carbide (B₄C)
9.5–10: Cubic Boron Nitride (cBN)
10: Diamond
Why the Mohs Scale is Less Precise for Super-Hard Materials
The Mohs scale is an ordinal, scratch-resistance scale from 1 (talc) to 10 (diamond). It’s excellent for mineral identification but becomes less discriminating at the very top. This is why materials with vastly different quantitative hardness values (like B₄C at ~38 GPa and diamond at ~100 GPa) can both be rated between 9 and 10.
For modern engineering ceramics like boron carbide, scientists and engineers almost exclusively use quantitative scales:
Vickers Hardness (Hv): ~30–40 GPa
Knoop Hardness (Hk): ~27–30 GPa
These scales apply a known force with a pyramidal diamond indenter and measure the size of the permanent impression. They provide precise, reproducible numbers for direct comparison.
The Bottom Line on Boron Carbide’s Hardness
Mohs Hardness: ~9.3–9.5 – This succinctly tells you it’s harder than sapphire (9) and nearly as hard as diamond (10).
In Practice: If you attempted a classic Mohs scratch test, a sharp, high-quality boron carbide crystal would scratch corundum (sapphire) with ease, would likely be able to scratch itself (same hardness), might leave a faint scratch on diamond or cubic boron nitride under high load, and would almost certainly be scratched by those two materials.
So, while “Mohs 9.5” is a perfectly valid and useful shorthand for its extreme scratch resistance, the more technically relevant fact is that boron carbide is the third-hardest known material, surpassed only by diamond and cubic boron nitride.