Small gems

Just to make it clear, you can give your spirit weapon nearly all small gems (small green gem, small blue gem, small red gem, and small silver gem, NOT small gem, since it's stackable). Green and blue small gems will not give exp, but they are a free way to satiate your spirit weapon. Red and Silver gems increase its Intelligence (the silver one gave 16 of Exp, compared to the 6 or 4 a normal weapon would have given)

“	NOT small gem, since it's stackable	„

Just because an item is stackable, it doesn't mean that it can't be fed to spirit weapons. ie. Fine Gem Powder

Yeah, but right now that's the only stackable item you can give, and it is already written in the page

Which is currently in the process of being rewritten. The way you said it initially made it sound like you couldn't feed it only because it was a stacked item; which is exactly what gem powder is (regardless of whether or not gem powder is the only kind of stacked item that can currently be fed). Also, the Spirit Weapon Item List already lists the gems.

Ah... ok, sorry :-(

As a general rule of thumb, you can feed an item to a spirit weapon iff the item is not stackable.

There are exceptions to this rule, but it still sets a fair expectation.