Bronze

Bronze is copper alloyed with tin, sometimes arsenic. Pure copper is soft. Tin makes it hard enough to hold an edge. The alloy spreads through Mesopotamia, Egypt, the Indus, and later Europe and China in the third millennium BCE.

The contribution is a tool metal you can cast. You pour it into a mold and get the same axe or chisel again. Stone has to be flaked one piece at a time. Repeatability is the new thing.

Bronze is expensive because tin and copper rarely sit in the same ground. Trade networks grow to move them. When iron arrives, it does not make bronze useless. Iron is cheaper and more common. Bronze stays for fittings, bells, and art.

Without bronze, the habit of smelting still exists in copper. Iron needs hotter furnaces than bronze did. The hotter fire is a step past this one, not a replacement of the idea.