Jewel

A jewel in a watch movement is a small bearing made from synthetic corundum, the same crystal as ruby and sapphire, pierced and polished to carry a wheel pivot or to take the sliding impact of the escapement. Jewels are functional parts, not decoration, and despite the name they are not precious: synthetic corundum has been produced industrially and cheaply since the flame fusion process developed by Auguste Verneuil was commercialized in the early 20th century.

The physical argument for them is friction and scale. Corundum is hardness 9 on the Mohs scale, harder than any steel pivot that will turn in it, and it polishes smooth enough that oil stays where it is put. The parts are very small: the pivots at each end of a wristwatch balance staff are on the order of a tenth of a millimeter across, and the axial free play left between the jewels, called endshake, is a few hundredths of a millimeter. A hole of that size worn oval in a brass plate is a dead movement; the same hole in corundum holds its shape for decades.

What they actually do

Four kinds do nearly all the work.

• Hole jewels are ring bearings for wheel pivots. The bore is usually olive shaped rather than straight, so the pivot touches a narrow band at the middle instead of the whole depth, which cuts friction and traps oil at the contact.
• Cap jewels, or endstones, are flat discs closing the end of a pivot hole. On the balance staff they keep the pivot located and hold oil in place by surface tension.
• Pallet stones are the two rectangular jewels set in the pallet fork that lock the escape wheel and receive its impulse.
• The roller jewel, or impulse pin, is the upright pin on the balance's roller table that the fork pushes against.

The balance jewels are also where shock protection lives. In an Incabloc, KIF, Seiko Diashock or Miyota Parashock setting, the hole and cap jewels sit together in a holder held by a lyre-shaped spring, free to move sideways and upward under impact and to return the staff to center afterwards. Without it, a dropped watch breaks the finest pivot in the movement, which is why this arrangement became universal after the 1930s.

How many a movement needs

Seventeen is the count for a fully jeweled simple hand-wound movement, and it is worth seeing where they sit: four at the balance (two hole, two cap), four at the pallet fork (two hole jewels and the two pallet stones), one roller jewel, and pairs at the escape, fourth, third and center wheels. Add automatic work and the number rises with the number of new pivots: the ETA 2824-2 has 25, the Seiko NH35 has 24, the Miyota 8215 has 21, the Valjoux 7750 chronograph has 25 and the Grand Seiko 9S65 has 35.

One comparison on this site makes the point better than any argument. The ETA 2892-A2 has 21 jewels against the 2824-2's 25, because its automatic work is flatter and uses fewer wheels, and it is the thinner, more expensive, more respected movement of the two.

Where they came from

Jeweled bearings were patented in London in 1704 by Nicolas Fatio de Duillier working with the Debaufre brothers, using pierced natural ruby and sapphire. For two centuries that made them a luxury, which is the origin of the idea that a jewel count says something about price. The Verneuil flame fusion process, commercialized at the start of the 20th century, grows corundum boules from aluminum oxide powder, and the cost of the raw material collapsed. A modern jewel is worth cents; the cost at the bench is the labor of fitting one and adjusting the endshake around it.

Common misconceptions

Jewel count is not a quality signal. Past the count the design actually needs, extra jewels do nothing, and counts have been inflated for marketing more than once in the industry's history: mid-century watches advertised 41, 100 or more jewels, with the surplus set into places that carry no load at all. Both the US Federal Trade Commission's guidance for the watch trade and the Swiss standard on counting jewels responded by restricting the countable number to jewels that perform a mechanical function. A very high number on a simple movement is a warning, not a boast.

Jewels are not consumables either. What ages is the oil in them, and different oils go in different places: a thin oil for fast pivots such as the escape wheel, a thicker one for slow, heavily loaded pivots, and a grease on the pallet stones. All of them thin, spread and oxidize on a timescale of years, which is the real reason for servicing. A jewel itself fails only when it cracks under a shock or when a worn pivot ovals the hole, both of which a watchmaker finds by inspection rather than by the calendar.

How to read the spec

Take the jewel count as a rough clue to what kind of movement you are looking at and nothing more. The details that matter are elsewhere: whether the balance jewels are shock-protected, how the pivots are finished, and whether the jewels are pressed straight into the plate or set in chatons, the small metal collars used in high-end work for tradition and appearance as much as for serviceability. Our guide to reading a watch specification puts the number in its place alongside the ones that do carry information, and watch jargon decoded covers the rest of the spec-sheet folklore.