Quote Originally Posted by Fouredged Sword View Post
It wouldn't resist heat or cold well. In fact, it would have a tendency to ignite and burn when hit by high intensity flames. Then you are basically wrapped in a burning tomb of coal.
Furthermore, it's crystal, so using standard D&D logic, it probably wouldn't do well against sonic attacks.

On to the positives. Obviously, it's not metal, so effects keyed to metal (Heat Metal for example) wouldn't work on it, and certain effects that gain bonuses against metal wouldn't gain those bonuses. A kineticist firing an Electrical Energy Ray at the diamond armor-wearing BSF wouldn't get the +3 bonus.
You might also be able to argue that it could have some kind of ray deflection abilities.