I have no idea if Apple was using Gorilla Glass on the back, nor did I know whether GG is harder or softer than sapphire. I'd assume softer, since it's just a glass, right?

http://www.corning.com/docs/specialtymaterials/pisheets/PI23... gives its Vickers hardness as 701 kgf/mm^2, which http://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness#... would seem to place in the 5-6.5 range of the Mohs scale, nowhere close to 9. However, Vickers is a scale of resistance to plastic deformation, while Mohs is a (highly nonlinear!) scale of resistance to abrasion, so perhaps take that with a gram of potassium chloride.

Other random places suggest sapphire's Vickers hardness is around 1800-2200 kgf/mm^2, which is substantially harder than the number Corning's brochure gives for Gorilla Glass.