Orichalcum is a mythical metal described in several Ancient Greek texts, whose exact composition has never been determined.
Although it was mentioned without explanation in some earlier poems, Orichalcum most famously appears in Plato's discussion of the mythical lost island of Atlantis. In Plato's dialogue Critias, Critias mentions that the outer walls of Atlantis were plated with orichalcum, and that there were several orichalcum mines on the island. He describes orichalcum as being second only in value to gold, and describes its appearance as "fiery red" on more than one occasion, but mentions that nowadays the metal "exists only as a name."
The word "orichalcum" literally means "mountain copper" and comes from the Greek roots όρος (oros, "mountain") and χαλκός (chalkos, "copper"). The Romans mistranslated this term into the Latin folk etymology aurichalcum, meaning "gold-copper", and sometimes used the term to refer to common alloys of gold and copper.
In modern times, the term "orichalcum" is used in three contexts: to describe the mythical Atlantean metal, often deemed to have magical properties in fantasy fiction and role-playing games; by numismatists to describe the gold-colored bronze alloy used to make the Roman sestertius and dupondius coins, and as an alternative term for the gold-copper alloy tumbaga, used by natives of the Andes mountains, which has been employed as "evidence" in extravagantly imaginative arguments that Atlantis was actually South America.
However, the true nature of the ancient orichalcum, or if it ever actually existed outside of poetry and fiction, has never been established. Theories have included everything from a type of brass to a gold-silver alloy or gold-copper alloy to some hitherto undiscovered metal to a type of amber to primarily a figment of Plato's imagination.
Given that we still don't know 2,500 years on, we will probably never know, but the cool-sounding name, the ties to Atlantis, and the mystery of it all has ensured that orichalcum is enshrined up there with mithril and adamantium as a perennial favorite of sci-fi authors, role-playing gamers, and manga and anime creators the world over.