Stoneskin turns flesh as hard as stone, not into stone. Technically it doesn't need any kind of visual effect. It needs a CLANGG sound effect as weapons bounce off the hardened skin, and a *resisted* indicator.
I can only hope Larian can resist the urge of applying a literal stone texture all over the character model just because they can and the spell has the word "stone" in it.
A good visual effect would be a wave of grey turning the character's skin to stone from head to toes, but then after a second fading back into the character's normal skin tone. This would give a nice visual that the spell has been activated but then not be a permanent visual efffect.
