ceramic skateboard bearings

The ceramic skateboard bearings are typically hybrid ceramic bearings, meaning the inner and outer rings are made of steel or chrome, but the rolling balls are made of a ceramic material. The ceramic material typically used in skate bearings is Silicon Nitride (Si3N4). Bearings made of 100% ceramic are also available, but they are typically used in electric motor applications because of their nonconductive properties. 

Definition of "Ceramic" 

Any of various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing a nonmetallic mineral, such as clay, at a high temperature. 

Manufacturer Claims 

Here are some of the claims made by the manufacturers of ceramic bearings, but note that some of these claims were made for hybrid ceramic roll ball bearings (the same type used in skates) that are used in manufacturing equipment, so I don't know if the same claims would be made when those bearings are used in skates.

60% lighter than steel balls

Centrifugal forces are reduced

Lower vibration levels

Less heat builds up

Reduced ball skidding

Fatigue life increased

50% higher modulus of elasticity

Improved spindle rigidity

Naturally fatigue resistant

Tribochemically inert

Low adhesive wear

Improved lubricant life

Superior corrosion resistance

Bearing service life is two to five times longer

Running speeds are 50% higher

Overall accuracy and quality improves. Better workpiece finish characteristics

Lower operating costs

Boost productivity

High temperature capability

Cutting tool life is increased Ceramic bearings, hybrid, are constructed of steel inner and outer rings with ceramic (typically Si3N4) balls in place of steel.