Jewel bearing

Jewel bearings are used in precision instruments where low friction, long life, and dimensional accuracy are important.

[2] The advantages of jewel bearings include high accuracy, very small size and weight, low and predictable friction, good temperature stability, and the ability to operate without lubrication and in corrosive environments.

[3] Disadvantages include brittleness and fragility, limited availability and applicability in medium and large bearing sizes and capacities, and friction variations if the load is not axial.

Manufacturers traditionally listed the number of jewels prominently on the watch face or back, as an advertising point.

In such instruments, jewel bearings are often used as pivots for their needles which need to move reliably and with low variability even when measuring small changes.

Ruby jewel bearings used for a balance wheel in a mechanical watch movement
Cross-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.