In horology a jewel is a small piece of synthetic corundum, almost always synthetic ruby, that is precisely shaped, drilled, and set into a watch movement to act as a friction-reducing bearing. Unlike a gem-quality stone valued for its appearance, a movement jewel is a precision component selected for its hardness, its dimensional stability, and its ability to retain oil at the pivot interface. The component is invisible in normal wear but is fundamental to the long-term operation of a mechanical timepiece.

Origin

The use of pierced ruby bearings in watches was pioneered in 1704 by Nicolas Fatio de Duillier, working with watchmakers Peter and Jacob Debaufre under a fourteen-year English patent. Their proposal solved a practical problem of the period: brass plates running directly against steel pivots wore unevenly and required frequent overhaul. Natural ruby and sapphire offered the necessary hardness, with a Mohs value of 9, and could be drilled by diamond bowstring tools to form an open hole jewel.

Through the eighteenth and most of the nineteenth century jewels were cut from natural corundum, which limited the size of the trade and contributed to the high cost of fully jewelled movements. The introduction of the Verneuil flame-fusion process in 1902 by Auguste Verneuil produced synthetic corundum boules at industrial scale and at costs an order of magnitude lower than natural stone. By the mid-1920s essentially all watch jewels were synthetic, a transition that made the seventeen-jewel lever escapement an affordable standard.

Forms

Several distinct forms of movement jewel are recognised. The hole jewel is a flat or slightly domed disc with a central polished hole, mounted in the plate or bridge as a radial bearing for the pivot of a gear-train wheel. The cap jewel is a small plain disc set above an end pivot to close the bearing, retaining the oil charge and limiting end-shake. The pallet stone is a rectangular block, polished on the impulse face, set into the pallet fork of the escapement. The impulse jewel, sometimes called the roller jewel or ruby pin, is a small cylindrical or D-shaped pin set into the balance roller that transmits energy between the pallet fork and the balance wheel.

Manufacture

Modern jewels are produced by sectioning Verneuil boules into wafers, profiling those wafers on diamond grinders, drilling the central hole with diamond paste, and polishing the bearing surfaces to a final finish that approaches optical flatness. Tolerances are held to within a few microns. Major suppliers historically included Pierres Holding (now part of the Swatch Group) and Sandberg in Switzerland, with smaller specialist works in Germany and Japan.

Service Considerations

Jewels themselves rarely fail, but their oil charge degrades over time, gumming the bearing and increasing friction. Standard service intervals of four to seven years are based on the working life of the lubricant rather than the jewels. A cracked jewel, which usually results from impact or from a pivot being driven against the hole at high speed, requires replacement in the bench rather than refinishing. The set jewel uses a brass collet and is friction-fitted, while the modern shock-protected jewel is mounted in a sprung housing, of which Incabloc, Kif, and Etachoc are the main commercial systems.