Stoneskin for RDM/WHM/SCH (the spell) was a flat value shield placed on the caster based on MND and enhancing magic skill (with a soft cap). There wasn't any healing attached to it.

WHM was the only one who got "Cure-skin".

I kinda wish AST had a similar mana regeneration mechanic similar to sublimation from SCH in ffxi. I always was a fan of it.