Small gems

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.