A bonding agents added to metal to increase their strength
Rheanium is generally not found in nature and it is argued that the substance is either manufactured or the resulted of artificial substances. It has only been found beneath the remains of ancient civilizations giving weight to the theory that it is not a naturally occurring substance, as some races have figured out how to manufacture it.
Rheanium works by bonding with metal substances and filing in the gaps between molecules. This increases strength and density, in some cases, it would take a fission device or stronger to damage it. However, Rheanium is not the easiest thing to produce.