Orichalcum is a metal mentioned in several ancient writings, including the story of Atlantis as recounted in the Critias dialogue, recorded by Plato. According to Critias, orichalcum was considered second only to gold in value, and was found and mined in many parts of Atlantis in ancient times. By the time of Critias, however, it was known only by name. In numismatics, orichalcum is the golden-colored bronze alloy used for the sestertius and dupondius coins. In many sources of pop culture, such as novels and video games, orichalcum is presented as a valuable ore that can be mined and crafted into powerful armor and weapons.
The name derives from the Greek ὀρείχαλκος, oreikhalkos (from ὄρος, oros, mountain and χαλκός, chalkos, copper or bronze), meaning “mountain copper” or “mountain metal.”
The Romans transliterated “orichalcum” as “aurichalcum,” which was thought to literally mean “gold copper.” It is known from the writings of Cicero that the metal they called orichalcum, while it resembled gold in colour, had a much lower value.
Orichalcum has variously been held to be a gold/copper alloy, a copper-tin or copper-zinc brass, or a metal no longer known. The Andean alloy tumbaga fits the same description, being a gold/copper alloy. However, in Vergil’s Aeneid it was mentioned that the breastplate of Turnus was “stiff with gold and white orachalc” and it has been theorised that it is an alloy of gold and silver, though it is not known for certain what orichalcum was.
In later years, “orichalcum” was used to describe the sulfide mineral chalcopyrite or brass. However, these are difficult to reconcile with the text of Critias, because he states that the metal was “only a name” by his time, while brass and chalcopyrite continued to be very important through the time of Plato until today. For that reason, other authors on the subject[who?] conclude that orichalcum is either the gold-copper alloy tumbaga, or possibly amber.
There have been references that Orichalcum was in fact electrum by another name, and that through selective casting the metal smith could alter the amounts to vary the colour. Other references mention that a liquid metal was added possibly mercury (so a mix of gold, silver, copper and mercury), and that the alloy was heated while lighting in a jar was added (possibly a early reference to electroplating or low level electricity like the Baghdad Battery being applied, while the metal was heated.)