Jewels are small bearings in a watch movement: a metal spindle turns inside a pivot hole lined with a hard stone. Most are synthetic sapphire or synthetic ruby. They are valued because their kinetic friction is low and their static friction is very consistent. They are found mainly in mechanical watches.
How does a jewel bearing work?
A jewel bearing is a plain bearing. It has no balls or rollers. A metal spindle, or pivot, sits in a hole, and the hole is lined with a jewel. The spindle turns against the smooth, hard stone. Two things make this useful. Kinetic friction is the drag on a part that is already moving, and in a jewel bearing it is low. Static friction is the resistance a part must overcome to start moving, and in a jewel bearing it is very consistent. So the parts turn easily, and they behave the same way each time.
Where it comes from
Jewel bearings for watches were invented in 1704 by Nicolas Fatio de Duillier, Peter Debaufre and Jacob Debaufre. The three inventors were granted English patent number 371 for the idea. The first watch jewels were natural stones. Diamond, sapphire, ruby and garnet were all used. Modern jewels are different. Most are now lab-grown sapphires or rubies. Both are forms of corundum, and both rate 9 on the Mohs hardness scale.
What does the jewel count mean?
The jewel count is the number of jewels used in a watch movement. A typical fully jewelled watch that only tells the time has 17 jewels. Watchmakers have traditionally used the count in advertising. It is often printed on the dial or on the case back. The figure matters most for mechanical watches, where jewel bearings are mainly used. Many quartz movements do without jewel bearings at all.
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