Don't say Nu Metal, or Glam, because there are worse genres.
The first metals that people used were actually discovered by accident. People in ancient Mesopotamia saw molten copper and lead on the ground where they built their fires. They learned that the metal has useful properties and cast tools in the ground, using small drainage ditches from the fire pits. (we're talking inches here!)
A furnace doesn't have to be made of metal, you can extract metals in a furnace made of brick or stone.
Furnaces are made out of stone, not metal. Some metals do not need refining. gold, silver can all be found lying on the ground in their metallic state. Metals like copper and zinc can be refined with just a small amount of heat. And iron refining can be done without any metal at all. .
Metals were formed in supernovae explosions. During accretion (how our planet formed), some metals and gas condensed to form the planets. In the center of the Earth, convection currents and a heat greater than any furnace assisted in combining like elements and bringing it to the surface of the planet via volcanoes...etc People could literally pick it up in the past. As time progressed they made clay furnaces to smelt the metal from ore. The more they smelted, the more they had... the larger the furnace. Even modern furnaces are lined with brick even today.
