Premium ABEC-5 Stainless Steel Ball Bearing with Si3N4 Ceramic Balls (Silicon Nitride) 4mm x 8mm x 3mm 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 bearings live at least 5 times more than any other bearing.