Aside from carbon, what other substances can be made "superhard"?
Interestingly, nobody addressed the reason why diamonds are hard in the first place. The pressure (and temperature) are not the reason why they're hard, only the reason why they are formed. The diamonds are hard because the carbon atoms are bonded together by sigma ($sp^3$) bonds, which are the strongest chemical bonds. Other materials exhibiting the same kind of bonding are potentially also very hard. But the small atomic number of carbon (Z=6) means that the bonds will be stronger than in higher atomic number atoms, such as silicon.

However, amorphous (non-crystalline, like glass) materials can also be extremely hard. There is even an example that is even stronger than diamond (although not naturally occurring). And this material is made of... carbon! Exactly like diamond. This material, dubbed Q-carbon, was discovered in 2015, and is even created at atmospheric pressure, and around 4000 K with the help of a laser pulse. And it is estimated to be 10-20 % harder than diamond.

Bottomline: gigantic pressures are not required to create extremely hard materials, and diamonds is no longer the strongest known material.