As it being naturally the hardest mineral in the earth (and in real-life), it can withstand shots from the strongest cannon type, Explosive type.

However, it comes with a very disappointing downside, its malleability is very low and the ore for it is also rare, that the only real use for it is studded armor.

Note: Diamond studded armor is not completely resistant to Explosive cannons. The diamond is resistant, the wall behind it isn't. Sorry :P