US Patent & Trademark Office → AppleInsider: Apple’s patent application titled “Nitriding Stainless Steel for Consumer Electronic Products”:
In addition to providing a durable, hard surface that is both scratch and impact resistant, the nitride layer allows for the natural surface color and texture of the underlying stainless steel to remain visible to the user. It is this natural surface color and texture of the stainless steel that adds to the aesthetically pleasing appearance of the consumer electronic product, thereby enhancing the user’s overall experience.
I’m not too worried about stainless steel getting scratched. How about a nitride coating over glass?