[dhx]

Diamond is only the hardest known naturally occurring material.

For the hardest known material, take a look at this paper from 2005: http://adsabs.harvard.edu/abs/2005ApPhL..87h3106D

Or the Wikipedia article for a more general treatment: https://en.wikipedia.org/wiki/Aggregated_diamond_nanorod

[gliese1337]

Not even that, quite. There are two known naturally occurring minerals that are harder than diamond: wurtzite boron nitride, and lonsdaleite (another carbon allotrope). They just don't occur in very large quantities.

Ref: http://www.newscientist.com/article/dn16610-diamond-no-longe...