Robinson stuff is color-anodized and that may have come off after a while, mine did but the pinion is still perfectly good. Other than that, the material is light-years harder than even the best plastic spur, which means you probably picked up sand.
If you need a super-pinion, steel pinions are available from Hobbyking.com, at reasonable prices as well... but you would need to buy a good number of them to counter the shipping cost from Hong Kong.