kupro

The metal is named after an island. Latin called it aes Cyprium, the metal of Cyprus, because that is where the empire's supply came from. Speakers dropped aes, the phrase collapsed to cuprum, and every European name for the metal - copper, Kupfer, cuivre, kobber - is a worn-down piece of an address. Kupro and Cyprus are the same word, and the chemical symbol Cu is the Latin still sitting there in the periodic table.

Where it sits in the site's metal set. You now have oro at 374, arĝento at 798, fero at 1,104, the general metalo at 1,777, ŝtalo at 2,215, stano at 2,463 and now kupro. That is nearly the whole ancient toolkit, and the ranks tell you something true: the metals people handle daily entered the corpus early, the industrial ones late.

The alloy has its own root. Copper plus stano is bronze, and Esperanto does not build that from the two metals: bronzo came in whole, as it did in every other European language. Latuno is brass, on the same terms. What Esperanto does derive is everything else - kupra is the adjective for both the material and the colour, so kupraj haroj is copper hair with nothing further to explain, kupraĵo is a copper object, and a copper mine is simply a kupra minejo.