Jewels
The synthetic ruby bearings that reduce friction
What it is
Jewels in watchmaking are small discs or cups of synthetic ruby (aluminium oxide, corundum) pressed into brass settings (chatons) in the movement plates and bridges. Wheel pivots run inside hole jewels; pivot tips rest against cap jewels. Their hardness prevents wear at the bearing surface; their smooth, dense surface retains lubricating oil by capillary action without absorbing it. The standard fully-jeweled movement has 17 jewels; one for every bearing that takes significant wear.
History
Natural rubies were first used in watch movements by Nicolas Fatio de Duillier and Peter and Jacob Debaufre, who received the first jewel-bearing patent in London in 1704. The technique spread slowly through the 18th century; jeweled pivot bearings were considered luxury specifications. Synthetic rubies, grown by the Verneuil flame-fusion process (developed 1902), replaced natural stones from the early 20th century onward: cheaper, more consistent in hardness and dimensions, and available in any quantity. The mid-20th century saw a marketing war over jewel counts: manufacturers added non-functional jewels to inflate numbers beyond the 17 that cover every load-bearing pivot. The relevant benchmark is 17; anything above that in a standard three-hand movement is typically decorative or provides marginal benefit in specific high-wear locations. Anti-shock settings; Incabloc (introduced 1934), KIF (1944), and their successors; mount the balance wheel's jewels in spring-loaded holders that absorb impact and return the pivot to its correct position, protecting the most vulnerable bearing in the movement.
How it works
A hole jewel is a thin disc with a precisely-drilled hole at its center; the wheel pivot passes through the hole and rotates within it. A cap jewel is a flat disc against which the rounded tip of the pivot rests under endshake; the small axial play that prevents the arbor from binding. Oil is applied to the jewel surface during servicing; the capillary geometry of the jewel-pivot gap holds the oil in place between services. The jewel's hardness (9 on the Mohs scale, just below diamond at 10) means the steel pivot wears before the jewel surface does; and since pivots can be replaced but plates cannot, this wear hierarchy is intentional. Running a pivot against a jewel without oil accelerates pivot wear dramatically; correctly oiled jewels can run for three to five years between services in normal conditions.
In the catalog
Related
- Escapement: The mechanism that divides time into equal steps
- Gear train: The series of wheels reducing the mainspring's speed to readable time
