The (MF63ZZC) 3x6x2.5 Halo ceramic flanged series of bearings are a high-quality steel/chrome bearing with metal shields and silicone nitride ceramic balls (Si3N4). These bearings have been tested and proven by many racers.
A ceramic can spin at higher RPMs with less drag and the life of the bearing is far greater than a steel bearing but in our industry, we suggest that you maintain them with oil or grease every half-gallon to a gallon for the longest life possible. Another benefit to ceramics is the weight difference in the balls allowing for less rotating mass.