It works like unlocking the different races. Go to Havendale Bridge, find the subdungeon, talk to the transmuter and leave the dungeon alive. The transmuter doesn't ask you to do anything within the dungeon - unlike the race representatives. When you are back to the kingdom, you will be given a small quest chain, where you have to salvage golem hearts in several defined subdungeons. After that the transmuter class and the rock heart will be unlocked.