Hm.
Well, so far, all magic has come from Adonalsium. And with the possible exception of Shardblades/Shardplate/Dawnshards (about which we know very little), all magic has at least initially resided inside of people (Nightblood had 1000 breaths, so he still follows this rule).
So... I dunno. Objects can't gather power naturally, it has to be infused into them somehow?