Material

Rollerblade bearings can be made of different materials.
One type of material commonly used to make Rollerblade bearings is steel.
Another material commonly used is ceramic. With ceramic bearings,
the bearings can be held in place by either a steel retainer or a ceramic retainer,
depending on the design of the bearings.

Sealing

Rollerblade bearings can be either sealed or open.
Sealing the bearings keeps dirt, road grime and dust out of the bearing.
In some cases, the seal may be made of steel.
Other bearing designs use rubber on the side of the bearing to keep dirt out of the moving parts of the bearing.
The seal can sometimes be made of other materials, but sits between the inner and outer rails around which the bearings rotate.
In some cases, the bearings may come into contact with the seal. In other cases, the seal is designed to ride above the bearings.

Rating

Rollerblade bearings are rated using the American Bearing Engineering Council (ABEC) system.
The ABEC system measures precision. More precise bearings have less play.
This "tightness" allows them to spin faster with less friction caused by excessive motion.
The ABEC scale ranges from 1 to 9. ABEC 1 bearings are manufactured to lower tolerances than ABEC 9 bearings.
Some Rollerblade bearings are not rated. These are generally manufactured to even lower tolerances.
ABEC 1 bearings are generally cheaper than higher-rated bearings.
However, ABEC-9 bearings are generally more expensive than lower rated or unrated bearings.
ABEC 5 or ABEC 7 bearings often offer a good balance between price and performance.