Probably via Italian and re-loaned back into Greek during the Venetokratia. The same as σάπφειρος → zaffiro → ζαφίρι.TIL that diamond is from the greek term 'adamas', meaning (i suppose) non-tamable (due to how hard it is).
Wasn't conscious of that, since while the term adamantion exists as an adjective, we use the slight corruption 'diamanti' for the noun, where the root isn't as visible.