Corning® Gorilla® Glass is known for better durability, which is preventing the scratches from further propagating in comparison to a normal glass.
While stronger than normal, is still made of glass, and improper use can result in scratches and breakages. If the screen is still not looking normal, please see the repair options at your local Nokia support pages. Please note that liquid and physical damage are not covered by the warranty.
Nokia products with Corning® Gorilla® Glass are listed here .