I have a question about this...
When you allow Pokemon that evolve via stones to "hold" the evolution stones, will they evolve?
If not, my theory is that evolution stones aren't hold items. You actually have to give the Pokemon the stone. If they can be used as hold items, then Eevee's evolution is pretty messed up.
I don't really use stones that often so I don't know these kinds of things.
Also if you get it wet it will turn into a Gremlin because Eevee looks a bit like a Gizmo!
The thing is it that they are hold items as well as use items. O_o
A lot of items can be held, probably just for trading's sake. You can hold a potion or an antidote, but a Pokemon won't use it, because the Pokemon doesn't know how to use it. Think of it that way.