3 x 10 x 4 Stainless Steel Ball Bearings 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, also bearing is dry (not lubricated, to be smooth), Ceramic Balls are the highest standard of balls that can make your bearings live at least 5 times more than any other bearings.