A special metal that is extremely valuable, vastly more so than gold. Only a few dwarfs know how to craft items out of it. Physically, items made from it are virtually indestrible, less dense than water, ferrous, and poor conductors.
In addition to its physical properties, it has extraordinary mystical properties as well. It is the only substance revealed so far that Dungeon Hearts can not produce using gold. It is resistant to analysis from the Mercury Computer, and presumably other means as well. Physical items crafted from it are capable of harming spiritual beings normally immune to physical harm, including Dark Gods.
The Light Gods describe adamantine as 'not of this world', 'inimical to the forces of darkness', and being literally the congealed power of a (probably deceased) Light God. Oddly, they seem to state that adamantine grows over time, and imply that its presence within the earth may be necessary to keep the corrosive power of a deceased Dark God from harming the surface world. The Light Gods imply that adamantine is not quite indestructible, but so close that no known Dark God is powerful enough to break things made from it.
Readers sometimes refer to this metal as 'adamantium' but the story has consistently spelled it adamantine.