ToME: the Tales of Maj'Eyal

My Alchemist has a half equipped Golem : the thing wears a sword, an armour and a shield but there are two slots left and I can't find what items they are for. The icon looks like a book but... what does it mean ?

Those are gem slots and they start out locked.

You need to put a point in the golem gem skill from advanced golemancy before you are allowed to use the gem slots.