Maybe the Dwarves even created the Golem faction. Kind of an "I Robot" like thing where they made Stone Golems, Improved on them for Iron Golems, then tried to improve on them again but the result became sentient. Not content with being a "slave race" to the Dwarves these new Golems rebelled forming their own faction (and further improving themselves) and so to prevent it from happening again the Dwarves restricted golem creation to the more primitive Stone Golems and all information on more advanced golem creation was purged and further research strictly forbidden.

If/When the Golem faction is added you can then use types of metal for different golem types. Copper Golems, Brass Golems, Bronze Golems, Steel Golems, maybe even Mithril and Adamantine Golems