Knives cannot scratch true granite. The only thing harder than granite is diamond. For example, diamond is scored 10 on the MOH’s hardness scale and granite is scored 7. Diamond blades are the only thing that can cleanly cut through granite. Other natural stones like Gneiss and Schist are commonly called granite, but they do not have a MOH score of 7, and they can be probably scratched by knives.
Read more about preparing food on granite here.