I don't agree that stone-skin would protect from falling damage.
Here is an example that I hope will illustrate what I mean; stoneskin produces a rigid and hard covering over the body that resists blunt trauma etc. So it acts rather like plate armour. You try falling down the stairs in plate armour; does it protect you from falling? NO, in fact you would take more damage as your soft tissues squish in between the inflexible layers of metal (or stone in the case of stone-skin).
This is because, when you fall, your body has the energy (from gravity) and anything inflexible stopping that momentum reflects the energy back into your tissues.
When you get hit by a weapon, the energy is reflected back into the weapon so you take no damage. The two situations are totally different.