Hriston
Dungeon Master of Middle-earth (He/him)
I've seen some skepticism about that particular derivation. Could be a folk etymology. Like how amazon probably doesn't actually mean 'without a breast'.
Ah yes, I was also confusing etymology with meaning. So it could be derived from a Semitic language. Nevertheless, Wictionary gives three definitions for adamas, adamantos. Two relate to metals, and one is synonymous with diamond, which I think supports my point. Adamant isn't necessarily metallic. It could be made from some type of stone, possibly even diamond, and therefore be fair game for Druids if the prohibition against metal armor is taken at face value.