Here is Blocks grading system which I reference frequently. It’s geared a bit toward Handmades but I think it’s good. Hope this helps you!
We use the following grading system:
Mint (9.9-9.0). A marble that is in its original condition is identified as Mint. The surface has no marks or damage that occurred subsequent to being produced. There may be some minor rubbing on the surface of the marble is as it came from the factory. There may also be minor defects on the marble that occurred in production: popped air holes, melt marks, contact marks, annealing fractures. A marble that has factory markings will be graded as Mint(-) and the defects will be described.
Near Mint (8.9-8.0). A marble that has seen some usage is identified as Near Mint. The marble may have evidence of hit marks, including tiny to small subsurface moons, chips, flakes or bruises. The damage is apparent but does not detract from viewing the marble.
Good (7.9-7.0). A marble that has seen usage is identified as Good. The marble will have numerous hit marks, subsurface moons, chips, flakes or bruises. The core can still be seen clearly, but the marble has seen obvious usage.
Collectible (no grade, below 7.0). A marble that has seen significant usage is identified as Collectible. The marble will have moons, chips, flakes and bruises overall. The core is completely obscured in some spots. A Collectible marble has seen obvious extensive usage. It can serve as a collection placeholder until a better example replaces it.