Adamantine in D&D is traditionally portrayed as black, but IIRC, this is because it was originally the "drow metal", the dark elf equivalent of mithral. Etymologically, "adamantine" is related to "diamond" (you can see the same Greek root in both), but it didn't specifically mean "diamond", just "hardest possible thing" -- Perseus had an "adamantine" sword. Or maybe Minecraft is right and you can make arms and armor out of diamond!
Oh, and in case you're interested, orichalc is coppery red. (Because it was probably just copper.)