I don't have a Stealth but I do have some experience with DLC and other coatings. A plain 'ol stainless steel slide is going to scratch just like any other metal, so I believe your question is more about the durability of the DLC coating. You can use Google to learn more about DLC, but I will say it's a very hard coating, almost as hard as diamonds. I doubt it'll scratch but if you were to drop it I would imagine the metal under it would still deform and you'd see the dent.
There's another coating called Titanium Nitride (TiN) which is a direct competitor of DLC. I have TiN on my motorcycle's forks tubes (it's a gold color) and it's file-hard and extremely durable. You can't scratch it with a file but a direct hit from a rock will dent the metal under it. A gun slide that you can't scratch with a file is one tough finish!
I looked around and found some hardness values for some DLC applications and it also appears to be file-hard so I doubt you can scratch it. If it's really DLC on the Stealth then you shouldn't be able to wear it off either.
Perhaps some owners of R9 Stealth models will post back about durability of the coating. Hope this helps.
http://www.thegunzone.com/diamond_like_coating.html