Copper

Copper is a metallic chemical element, having the symbol Cu and the atomic number 29. It has had many uses in human history, including as a store of value (many cultures have minted copper coins), the substance of weapons and tools, and as the base of the alloy bronze.

Copper in "The Horse of Bronze"
Copper was a metal which was abundant in the centaurs' homeland. It could be alloyed with tin to create the alloy bronze, a very hard metal. Both the centaurs and the Sphinxes used copper for this purpose. When the centaurs' tin supply was disrupted by loss of contact with the Nuggies, they were forced to forge weapons of pure copper, which were much softer than the bronze weapons the sphinx soldiers carried. This put the centaurs at a significant military disadvantage, and in fact they lost a major battle because of it, which prompted them to send out several expeditions to attempt to restory their bronze supplies.