What is the difference between Sapphire and BK7? What is the difference between Sapphire and BK7 in optics (lenses), is it only about quality?
 A: No. The two glasses are made out of different materials: sapphire is aluminum oxide, and BK7 is silicon dioxide with some additives including about 10% boric oxide (Wikipedia). Therefore they refract light differently. Sapphire is also slightly birefringent, refracting differently polarized light waves differently. You can check refractiveindex.info for this information:
Sapphire (two graphs because it is birefringent)


BK7

A: Sapphire and BK7 have different optical indices and dispersion, but they also have different transmission windows. In short, sapphire will transmit some light in the infrared and ultraviolet, at wavelengths where BK7 would be opaque. That would be a typical reason to not use a glass such as BK7.
Sapphire is however birefringent, which is not a desired trait for lenses.
Sapphire will also have different mechanical properties. It is much harder for instance and will not usually get scratched. Both materials also have different behaviors when they get heated.
It's not possible to say that one material is better than the other, they have different applications.
