Thramnium is an extremely strong and resilient metal by weight, with the added benefit of a cumulative limited magical corruption resistance (it is able to absorb a certain amount of magical corruption during its lifetime with no ill effect).
Thaumium’s creation is a closely guarded secret. It is fabricated only by the Order Of Bone, and within the Order only the Order Of Marrow-Bleeding are taught the process of crafting it.
A common assumption among others in the order, based on the name, though not confirmed, is that it is somehow crafted of the bones of dead Manawracked.