orichalcum
A valuable yellow metal known to the Ancient Greeks and Romans; now sometimes interpreted as referring to a natural alloy of gold and copper, and sometimes treated as a mythical substance.
uncountable
Many walls were coated with metals – with brass, tin and a red ^([sic]) metal, unknown to us, called orichalcum.
Where it comes from
Learned borrowing from Latin orichalcum, itself a borrowing from Ancient Greek ὀρείχαλκος (oreíkhalkos), from dative singular of ὄρος (óros, “mountain”) + χαλκός (khalkós, “copper”).
Adapted from the Wiktionary entry orichalcum (CC BY-SA 4.0).