The Hardest Substance

Most recent answer: 10/22/2007

What is the hardest substance known?
- erika (age 10)
emmerdale high school, london

Erika -

The hardest substance known is diamond. [but see note below] Diamond is made up entirely of the element carbon. (Another thing that is made entirely of carbon is graphite, one of the softest substances.) Diamond's hardness is roughly 10.5-14.5 million psi.

The second hardest substance is a compound make from aluminum, magnesium, and boron. It was made for the first time in 1992 by scientists at Ames Laboratory in Iowa. Its hardness is 6.67 million psi (about half that of diamond). You can check out more at

Hard substances like these are very useful for things like drills, since regular drill bits will break if you try to go through something harder than they are. So when they want to cut through something /very/ hard, they use diamond drill bits instead. But since diamonds are so expensive, sometimes they use things like the aluminum-magnesium-boron compound I mentioned instead.


(published on 10/22/2007)

Follow-Up #1: new hard stuff

Please correct your answer to girl.
There is more than one substance. One of those would be known as "Ultrahard Fullerite C60".
- L.A.
We’re happy to pass this information along. See the Wikipedia article and references therein.


Mike W.

Lee H

(published on 10/22/2007)

Follow-Up #2: wurtzite and lonsdaleite

This is not a question, it is a correction. Diamond is the hardest naturally occurring substance found on Earth. But it is not the hardest substance. Wurtzite boron nitride (synthetic) and lonsdaleite (which comes from meteorites) are both harder.
- Marcy Seavey (age 39)
Waterloo, Iowa USA
Thanks for the tip. As nearly as I can tell, actual samples of those materials are not as hard as existing diamonds. There are some calculations indicating that defect-free versions of them should be harder than defect-free diamond. However, these are not very thermodynamically stable materials, so it is perhaps not surprising that no near-ideal samples have been found.

Mike W.

(published on 10/26/2011)