Why do watches have jewelry in them?
Wheels in a watch are constantly rotating while a watch is running. Wheels rotate on their pivots, which are thin posts on both ends of the pinion. This constant rotation causes friction, which in turn causes mechanical wear. To reduce friction and wear, the pivots rotate in synthetic sapphire jewels. Sapphire is used because of it’s hardness; only diamond is harder.
Jewels are cylindrical, pierced with a hole on the flat sides. One flat side has a concave oil sink, designed to retain oil on the pivots. One jewel is pressure-fitted into the mainplate, and the other into the bridge.
Different pivot sizes mean each set of jewels needs a different diameter hole. A center wheel rotates once per hour and is subject to high torque levels. Because of this, the pivot diameter is large. A fourth wheel rotates once per minute and is subject to low torque levels. Because of this, a small pivot diameter is used.
Shock-protecting jewel arrangements are used for balance wheels. Balance wheels have very thin pivots due to their oscillation speed. They are also very heavy, leading to increased stress on the balance pivots in the event of a shock. Shock-protecting jewels have a cap jewel mounted on a small spring to absorb the energy of any shock and protect the balance wheel pivots. Before the invention of shock-protecting jewels, broken balance staffs were a very common watch problem.
Before the use of jewels, mainplates and bridges were pierced directly. Without the use of jewels, the entire mainplate or bridge would have to be replaced when the pivot eventually caused mechanical wear. An improvement was to use bushings, which were easily replaceable, unlike the mainplate or bridge.
When first introduced, jewels were natural sapphire. Today, synthetic sapphire is used. Synthetic sapphire has the advantage of being less prone to cracking. It is also much cheaper to source and produce.