Many items in the game are made of metal including tools and weapons. There are many different variants of metal, each one with their own melting point. There are base metals which can be found in raw ore form in the ground, there are alloys that are made by combining the base metals in certain ratios, and then there are metals for which there is no current use. Any metal with a melting point at or below 1300°C can be melted down in a crucible but anything higher requires a bloomery.
Copper is very likely to be the first metal most players obtain. It can be found in chunks on the ground that give nuggets when broken, thus a metal hammer isn't needed. Charcoal is needed to smelt copper. Copper can be used to make every type of metal tool and armor. It is also used in the alloys of brass, tin bronze, black bronze, and bismuth bronze.
Iron is the best metal currently in the game. Unlike copper, it can't be found on the ground. It does appear in rocks and requires a tier 3 (Bronze) pickaxe to mine. A bronze anvil is also needed to work iron. Iron can only be smelted in a bloomery because of its high smelting temperature. Iron makes the highest tier tools and armor.
Lead is used to make molybdochalkos and has the unique property that it can be worked cold, but this has no use as the only item you can smith are plates, which are not used to craft anything.