Most likely, the grates have not chipped. It is usually metal from your pots and pans rubbing off on the grates. There are two ways to clean this: one is to use a brass brush and water. This will remove the metal markings. The other is to apply Cook Top Cleaning Crème to the grates use the abrasive side of a sponge. Finish by rinsing well. Both of these ways should return your grates to their original color.