Jewel
A synthetic ruby bearing used to reduce friction at pivot points inside the movement.
What are Jewels in a Watch?
Watch jewels are small, precisely drilled synthetic rubies (aluminum oxide, or corundum) used as bearing surfaces for the movement's rotating and oscillating parts. Because synthetic ruby is extremely hard and can be polished to a near-frictionless finish, it dramatically reduces wear at the pivot points where metal shafts spin at high speed.
Why Ruby?
Natural rubies were used in watches as far back as the early 18th century. Today all watch jewels are synthetic — grown in a laboratory — but the material remains the same. The hardness (9 on the Mohs scale, second only to diamond) and the ability to hold a tiny film of oil make ruby ideal for bearing surfaces.
Jewel Count
You will often see a movement described as having a certain number of jewels — "17 jewels," "21 jewels," and so on. A standard fully-jeweled movement has 17 jewels, placed at all the critical friction points. Higher counts (19, 21, 25+) add jewels at additional locations or include anti-shock settings. Counts above 21 in vintage watches were sometimes marketing rather than engineering — additional jewels placed where they add no functional benefit.
Jewels and Servicing
During a service, each jewel hole is inspected for chips, cracks, or excessive wear, and cleaned thoroughly. Old lubricant that has dried in the jewel holes is one of the most common causes of a sluggish or stopped movement.