Summoned Heatstone not Range Equippable
 

There is a picture of it being equippable on the wiki. Also I played an erudite mage on live and I quite distinctly remember using one in that slot every time i logged on.

Pretty sure it was the ammo slot?

hmm you might be right. It's definitely one of those. Anyway the current version is neck/prim/sec. Dunno about coldstone b/c my mage is only L16.