Bearings used are generally Grade 25 which are accurate to 25/1,000,000 of an inch. Less expensive ball bearings maybe grade 300, which are less round, at 300/1,000,000 of an inch. ... Hardness scale is measured in Rockwell, a steel ball-bearing is likely to be over a Rockwell C of 60. Ceramic ball bearings can be a Rockwell C of 75, much harder than the steel races, although I've not seen any Ceramic in bicycles yet, I do use them in industrial applications though & they are considerably more expensive! I always go to Swan Industrial Drives for mine, they don't generally supply any cheap stuff!