A fact to keep in mind is that metals may be randomized in the future, and civilizations may not have knowledge of all of them, so there's no narrative reason to have a backstory as to why dwarves know adamantine.
That would be deeply stupid. You're talking about randomizing chemistry. "Well, in this world the atmosphere is toxic to all forms of life, so you don't get to play as anyone." Woohoo!
Yes, that's exactly what I said.
Edit:
RANDOMIZATION AND RAWS ARC: More things need to make it out to the raws, and when they arrive, there should be some ability to randomize what you get in play. Current raw objects such as creatures, entities, plants,
metals or even stone should also be randomizable. This doesn't just include randomizing an existing entry, but, more importantly, the creation of entirely new objects. This behavior should be controlled by the world parameters so that you can run anything from a stock universe to a complete replacement of all existing raw objects. In worlds with random objects, the random objects need to be introduced to the player in such a way that familiarity can be gained (involving descriptions, back stories, etc.) so that it isn't just a mess of static with garbage names. Related to Core53, Core92, Core93, Core94, Core95 and Core96.