ZERØ ceramic bearings have ceramic balls and steel rings. The silicon nitride (Si3n4) balls are grade 3 and are the roundest balls you can buy without a military license. The rings are treated with a black oxide coating, which helps prevent rust in the bearing. The races are then touched up to a mirror finish after this treatment. Finally, they are cryogenically treated for strength before assembly. We also use VV silicone seals, which is the lightest seal available on the market. Dimensions: 17x28x7