What part of a self-winding movement is sometimes constructed from ceramic material?

In self-winding movements, the ball bearing of the rotor is often constructed from ceramic material due to the unique properties of ceramic. Ceramic is valued in watchmaking for its durability, lightweight characteristics, and resistance to wear and corrosion. When used in the ball bearing of the rotor, ceramic helps improve the efficiency and longevity of the movement. The rotor's role in a self-winding mechanism is crucial as it moves to wind the mainspring as the wearer moves their wrist. Thus, employing ceramic in this part greatly enhances performance by reducing friction during this movement, which can contribute to the overall reliability and accuracy of the watch.

Other options, such as the casing and the mainspring, typically do not use ceramic due to their requirements for specific mechanical properties like flexibility and strength under tension. The dial, while it can be made from various materials for aesthetic purposes, does not involve the functional requirements that the ceramic ball bearings fulfill in the movement.