You are confusing a fantasy game with real life. Like the book I was reading 5 minutes ago, the world is controlled by Dust, by your logic, that MUST mean the dead skin dust. The logic of assuming they mean the real world meaning is ridiculous, when it's so clearing a FANTASY game.
You're prove is a persons opinion.......several things wrong with that but moving on... the first 3-4 people say it's metal, last 1-2 say diamond.
The flaw with the idea of it being diamond is no one in their right mind would make armor or weapons out of diamond, it has no tensile strength at all, it'd just shatter. It's structure is the hardest in the world, doesn't mean lots of it is great protection.
Also if it was a diamond, why do all the armor pieces look very metal-ish?
If the Deathwing in this thread has diamond armor, the power of demon soul is unable to be contained and it killed him, with team fantasy lifting a finger, since diamond has no magical properties to contain the vast energies of demon soul.