Copper is reddish-colored metallic element found deep in mines throughout Earth.
Copper is a vital constituent of Romulan and Vulcan blood hemoglobin, causing their blood to be colored green. (TOS: "The Man Trap")
The water of a Class M planet surveyed by Nog and Jake Sisko in 2370 contained traces of nickel, copper and birythium. (DS9: "The Jem'Hadar")