Metallurgy
Metallurgy is simply, the art and science of working metal. It is the various breakthroughs in metallurgy which mark the boundaries of various archaeological ages.
The discovery of how to cast bronze was developed at the end of the Neolithic age, and it was widely used as the main metal for weapons or tools of the Bronze Age Mediterrainean and European peoples. By the time of classical Greece, the skill of forging iron had been developed, marking the beginning of the Iron Age.
Medieval Europe had an Iron Age level of development, as did most civilizations in Europe, Asia and Africa in period. The civilizations of the Native Americans and Polynesians did not have metallurgy at all, except for some limited use of gold and copper as decorative objects.