So as there are many things within the world that have a hardness, such as wood, steel and so on. What would the hardness of skin be? Bone is already given with the subject of special materials.
Rationale (probably unneeded):
With the acceptance of my DM, I've been allowed to use Gloves of Shaping as a torture device of sorts. Whether or not that's ethical...well I'll leave that up to you to decide. But it is a question I've had before, since it's something I've wanted to know not only revolving around me being able to use Gloves of Shaping on an Elf's face.