What is "jewel bearing"

n. a bearing, used in watches, gyroscopes and other instruments, normally made of a synthetic corundum

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 jewel material is usually some form of synthetic corundum, such as ruby. Jewel bearings are used in precision instruments where low friction, long life, and dimensional accuracy are important, but their largest use is in mechanical watches.