Orichalcum
Plato wrote about orichalcum in his book Critias.  In the book, Plato says orichalchum was in the ground of the legendary island of Atlantis.  To the Atlanteans, it was the second-most-valuable metal, after gold.  Plato says the Atlanteans put orichalcum on their city walls and temples.  Plato calls orichalcum "only a name" in his own time, meaning he did not think he had ever seen orichalcum.