American Classic say:
"Our fully sealed Ceramic cartridge bearings are ABEC-5 and use Silicon Nitride (Si3N4) ceramic balls with 440C stainless steel races. The Ceramic bearings produce an unparalleled ride feel in terms of smoothness, quickness and effortless rolling."
The MTB Ceramic Bearing Kit contains 6-6803C3 Ceramic Bearings.
The Road Ceramic Bearing Kit contains 2-688 and 4-6803C3 Ceramic Bearings (Micro 58)