So, what it "orichalcum" ?

In the legend of Atlantis, orichalcum is a reddish metal that sparkled like fire, mined in Atlantis but known only by name after the island's disappearance. According to Plato the mineral was found in many parts of Atlantis, being considered second only to gold in value. The three outer walls of the temple to Poseidon and Cleito were clad respectively with brass, tin, and the third, which encompassed the whole citadel, "flashed with the red light of orichalcum". Some sources call orichalcum mountain-brass or gold-brass. Perhaps it was copper or a bronze. In Latin it is called aurichalcum; the word is derived from the Greek word ορειχαλκον, which may be transliterated as oreichalkon.