Why is diamond called diamond?

According to Wikipedia which quotes LidDELL, H. G; Scott, R -- Adamas -- in a Greek-Enbglish Lexicon for the Perseus Project:

"The name diamond is derived from the ancient Greek αδάμας (adámas), 'proper.' 'unalterable,' 'unbreakable,' untamed', from ἀ- (a-), 'un-' + δαμάω (damáō), 'I overpower', 'I tame'."