The Apple Series 2 Watch [Stainless Steel] has Sapphire Crystal glass on the face of the Watch. It is on the Stainless Steel version only, not on the Aluminum version. It makes it nearly impossible to scratch the face/glass on the Watch.
Information on Sapphire Crystal Glass is as follows: [per online search/information]
Sapphire crystal is used by the high-end watchmakers. Sapphire scores a solid 9 out of 10 on the Mohs mineral hardness scale, just below diamond, making it even more scratch resistant than mineral glass. ... Even though sapphire is harder than mineral glass, the extra effort is worth the payoff.
I hope this answers your question.