Bronze
Bronze is made by melting two different metals and mixing them. The two metals are copper and tin. Copper, by itself, is too soft to make tools of (have you ever had a copper bracelet? It dents every time you knock your hand on a door). Tin is too brittle: it breaks too easily. But if you mix a little tin into the copper, it becomes bronze, which is much harder and at the same time less brittle. It is more useful for tools and also better for making statues.
When bronze gets old, and the air touches it, it corrodes (like iron rusting) and turns green, like these Etruscan greaves (leg armor):
Etruscan greaves (Vatican Museum, Rome), about 600 BC
