Scientists Discover Material Harder Than Diamond

Off the top of my head, I would say it's because the atoms are small and they form lots of bonds, carbon in particular. Diamond is a tetrahedral structure and I think the other really hard substances are also,

If you look in a periodic table, you will find that Boron, Carbon, and Nitrogen have 5, 6, and 7 electrons altogether. Any element with fewer valence electrons will be ineffective in forming latices with covalent bonds (as they have too few valence electrons). O and F are ineffective because they have too many valence electrons to effectively form a lattice as they can only form 1 or 2 valence bonds. Ne is a noble gas so its out of the running. All the rest can form nice valence bonds but the extra electron shells and increased atomic radius means that these valnce bonds are not as strong as the ones for B, N, and C.

Note: That the best explanation that I could come up with with my very basic knowledge of chemistry. I am sure a chem prof. will be able to give a much better answer.

There are quite a few uses listed in the last paragraph of the article.