Ruby jewel bearings used for a balance wheel in a mechanical watch movementCross-section of a jewel bearing in a mechanical watch. This type of donut-shaped bearing (red) is called a hole jewel, used for most of the ordinary wheels in the gear train. It is usually made of synthetic sapphire or ruby, press-fit into a hole in the movement's supporting plate (grey). The cup-shaped depression in the top of the jewel is the oil cup; its purpose is to hold the lubricating oil (yellow) in contact with the bearing shaft by capillary action.In wheels where friction is critical, a capstone is added on the end to prevent the shoulder of the shaft from bearing against the face of the jewel.
A jewel bearing is a plain bearing in which a metal spindle turns in a jewel-lined pivot hole. The hole is typically shaped like a torus and is slightly larger than the shaft diameter. The jewels are typically made from the mineral corundum, usually either synthetic sapphire or synthetic ruby. Jewel bearings are used in precision instruments where low friction, long life, and dimensional accuracy are important. Their main use is in mechanical watches.
