The 2x5x2.3 Ceramic Metal Bearing (682ZZC) is a high quality steel/chrome bearing with metal shields and silicon 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 once in a while. Another benefit to ceramics is the weight difference in the balls allowing for less rotating mass.