(covers information from several alternate timelines)
A warp plasma conduit made of copper
Copper is a reddish-colored metallic element that was found deep in mines throughout Earth. Zinc and copper could be combined into an alloy to form brass.
Copper was a vital constituent of Romulan and Vulcan blood hemoglobin, causing their blood to be colored green. (TOS: "The Man Trap") It was also a component in the blood of the Acamarians, along with iron, making Acamarian blood readily identifiable. (TNG: "The Vengeance Factor") It could also be used to produce copper tubing. (Star Trek: First Contact)
The water of a planet in the Gamma Quadrant surveyed by Nog and Jake Sisko in 2370 contained traces of nickel, copper and birythium, plus lead from the surrounding soil. (DS9: "The Jem'Hadar")
In an alternate timeline in which Captain Jonathan Archer and Crewman Daniels were trapped in the 31st century, Daniels had Archer take some copper which they had found and pound it into small strips no more than a millimeter thick for use in a device to enable communication with the past. (ENT: "Shockwave, Part II")