In numismatics, orichalcum is the name given to a brass-like alloy of copper and zinc used for the Roman sestertius and dupondius. Very similar in composition to modern brass, it had a golden-yellow color when freshly struck. In coinage, orichalcum's value was nearly double reddish copper or bronze. Because production cost was similar to copper or bronze, orichalcum's formulation and production were highly profitable government secrets.
In Natural History (xxxiii 95) Strabo (64 BC - 23? A.D.) wrote about orichalcum, There is a stone in the neighborhood of Andeira which when burned becomes iron, and then when heated in a furnace with a certain earth distills pseudargyros and this with the addition of copper makes the mixture called by some oreichalkos. Pseudargyros (Pseudo-silver) is presumably a term for zinc, while oreichalkos is orichalcum, brass.
The name orichalcum also refers to a fictional or legendary metal that is found in several ancient works of literature. Included among these is the story of Atlantis, written by Plato. This metal was said to have originated from mines in Atlantis and was the most valuable metal known by man, apart from gold. The use of the term orichalcum in this context should not be confused with the actual alloy used for Roman coinage.