S684ZZ Ceramic Premium ABEC-5 Ball Bearing Stainless Steel with Si3N4 Ceramic Balls (Silicon Nitride) 4mm x 9mm x 4mm S684ZZ Stainless Steel Ball Bearing with Ceramic Balls Premium ABEC-5 quality, Bearing is made of Stainless Steel, each bearing has 2 metal shields to protect the bearing from dust or any possible contamination, Ceramic Balls are the highest standard of balls that can make your bearing lives at least 5 times more than any other bearings.