4 x 10 x 4 Stainless Steel Premium ABEC-5 Ball Bearing with Si3N4 Ceramic Balls (Silicon Nitride), 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 bearings with Premium ABEC-5 quality.