Thanks!
Metal and Crystal can both be found in the mountain. Stone with yellow bits and stone with blue bits respectively. They take a lot longer to mine than normal stone. I tried to set this up so that you'd have to think a little bit about what your workers are mining at any given time, but on Normal difficulty it's not necessary to micromanage the mining. There also used to be obvious metal veins to follow, but I couldn't get the balance right and ended up removing enough of the metal from the map that it just seems sort of random.
The crystals were for an unfinished "tower upgrade ritual" feature. Maybe I should have taken the time to completely remove them from the game.