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Jewel bearing:

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Ruby jewel bearings used in an Omega mechanical watch movement.

Cross section of a jewel bearing in a watch.

A jewel bearing is a 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. In operation, the shaft tilts slightly so as to contact the jewel pivot hole at two opposite points.^{[citation needed]} The shaft rolls inside the bearing rather than sliding.^{[citation needed]} As the shaft rolls, the center precesses. Jewel bearings were invented in 1704 for use in watches by Nicolas Fatio de Duillier, Peter Debaufre and Jacob Debaufre who received an English patent for the idea. Originally natural jewels were used, such as sapphire, ruby, and garnet. In 1902 a process to make synthetic sapphire and ruby (crystalline aluminum oxide also known as corundum) was invented by Auguste Verneuil, making jewelled bearings much less expensive. Today most jewelled bearings are synthetic sapphire.

The advantages of jewel bearings include high accuracy, very small size and weight, low and predictable friction including good temperature stability, ability to operate without lubrication and in corrosive environments. They are known for their low static friction and highly consistent dynamic friction.^[1] The static coefficient of friction of brass-on-steel is 0.35, while that of sapphire-on-steel is 0.1-0.15.^[1]^[2] Sapphire surfaces are very hard and durable, with Mohs hardness of 9 and Knoop hardness of 2000, and can maintain smoothness over decades of use, thus reducing friction variability.^[1] Disadvantages include brittleness and fragility, limited availability/applicability in medium and large bearing sizes and capacities, and friction variations if the load is not axial.

The largest use for jewel bearings is in mechanical watches, where their low and predictable friction improves watch accuracy. A typical mark of watch quality was a note such as "17 jewels". More jewel bearings often meant better precision. Some makers added non-functional or unnecessary jewels to give the impression of accuracy. Some watches had as many as 100 jewels, most of them of no use. A typical "fully jeweled" time-only watch has two cap jewels, two pivot jewels, an impulse jewel for the balance wheel, two pivot jewels, two pallet jewels for the pallet fork, and two pivot jewels each for the escape, fourth, third and center wheels. Modern electronic watches achieve accuracy entirely separate from the friction of the mechanism, but early quartz watches used jewels to increase battery life, and high-grade quartz watches use jewels to reduce friction and wear.

Today, jewel bearings are also used widely in sensitive measuring equipment. They are typically used for very small applications such as high-precision instruments; galvanometers, compasses, gimbals, and turbine flow meters. Bearing bores are typically less than 1 mm and typically support loads of under 1 gram, although they are made as large as 10 mm and support loads up to about 500 g.^[1]

Historically, jewel pivots were made by grinding using diamond abrasive. Modern jewel pivots are often made using high-powered lasers, chemical etching, and ultrasonic milling.