Torchlight III

Metal is a raw material in Torchlight III that you can process within your fort and then use for the construction of a wide variety of buildable objects for your fort. Metal can be collected from ore deposits scattered all across the world, but is most common in underground area's such as caves or mines. The resource will show up on your minimap when you get close to it.

The 'Automaton' enemies can also occasionally drop metal, thank you 'The S Bot 9000' for the addition.

You do not need to have 500 metal on you at the same time for this achievement, you just need to collect 500 metal during your playthrough. It personally seems like it does not need to be on one character either, but if anyone can confirm (or deny) this that would be great, I will then adjust this solution accordingly.

The maps in this game are somewhat randomly generated when you sign into the game, so if you want to farm the same location for resources you can logout and simply login again to repeat the farming in the same location where all resources will be respawned.