Diamantanium is a hybrid synthetic metal that is the brain child of X-Man Mutant X. It is arguably the strongest and completely indestructible alloy.
During the 1990s, before he arrived at the Institute For Gifted Youngsters, Kevin Reynolds had discovered a large chunk of Adamantium in the a nearby mine where it landed several decades ago. He then took the substance and started tampering with it, leading him to discover that if it is combined with the hardness of a diamond, it will be completely indestructible. He successfully analyzed the substances and managed to synthesize it together. Once this happened, he realized he could place the substance into his body without any problem.
He intentionally modified his skeleton and claws with the substance which made him even stronger and made him somewhat more resistant to his physical vulnerability to magic.
Diamantanium is known to be thousands of times more stronger than Adamantium. It's powerful enough to withstand a heated Adamantium or even energized Adamantium blade. It's diamond hybridization makes it also the only metal to not have any form of magnetic attraction, which is why Magneto cannot manipulate or control it. This is caused by it's diamond half protecting it's metallic half. It's powerful enough to actually slice through Adamantium like a knife coursing through warm butter. The substance is so powerful that not even the might of Thor's mighty hammer could pierce it, nor could Galactus himself destroy it.
Diamantanium is completely indestructible and no other known person is known how to create or manipulate it other than Mutant X himself. Since it's conception, he's utilized the substance in many different things including his custom made hardware, tailoring and even furniture.