Adamant

Adamant is an olive-colored metal, the surface of which becomes highly reflective when exposed to magic but is normally a dull matte. It is incredibly rare, forming only at the intersection of the ethereal plane, the plane of earth, and the material plane, and only when elemental earth, stone, or metal passes through from the plane of earth to the material plane at the same time as a large amount of aether from the ethereal plane. The aether infuses the elemental material, forming "essential" material, or the magical essence of the earth, stone, or metal.
Essential metal is, in fact, the unforged form of adamant, and essential earth and stone, if they are not used for their own unique properties, can be smelted into adamant through a combination of extremely high heat and transmutation magic.
Once forged into a desired form, the application of a simple transmutation spell to harden the material permanently locks the metal into its current shape, turning it almost impervious to heat, breakage, or magic. Only another piece of adamant or powerful force magic can significantly affect adamant objects, and no magic can pass through a barrier made of it, making armor fashioned of the metal worth a king's ransom to military special forces and private adventurers. (Scry-proof chambers walled with paper-thin layers of adamant are also a common request, when large enough sources of the material are found.) The fact that adamant weapons can usually cut through any non-magical materials, and that bludgeoning weapons fashioned of adamant can create destructive vibrations in hard objects they strike, make adamant weapons almost as in-demand.
Non-military uses for the substance abound, as well, and only the extraordinary difficulty and danger of setting up a stable conflux of all three planes required to form the substance has prevented the metal from becoming ubiquitous in infrastructure and military use. While archmages are capable of forming stable planemelds for short periods, the cost is exorbitant and the amount of essential metal created per attempt is barely enough for a shortsword, making such endeavors a very niche market. Thus, the primary sources of adamant are rare house-sized deposits of essential earth found deep underground by prospectors, which when smelted can create a set of full plate or a few weapons, but are rarely used for non-military purposes.