Jewels are used extensively in vintage and modern watches due to their low friction and low wear characteristics. Low friction is vitally important, especially at the Balance Pivots, to reduce interference to the free motion of the oscillating balance wheel.
More modern jewels are made from a synthetic ruby, (aluminum oxide), and are extremely hard and long wearing. Even in vintage watches over 100 years old, seeing wear in the jewel hole is rare. There is a huge amount of frictional difference between a steel pivot running in a brass hole, versus a steel pivot running in a jeweled hole.
Another advantage of synthetic jewels is that dirt cannot be embedded in the hard surface of a jewel so the wear on pivots is greatly reduced. In a non-jeweled pivot hole, dirt can get embedded in the softer brass pivot hole and is one of the main causes of wear on the steel pivot and brass pivot hole changing a round hole to an oblong shape.
The jewel is basically a bearing, in which the wheel pivot rotates. The jewel is pressed into the plate of the watch, either the main-plate, bridge, or cock. In older pocket watches the jewels are burnished or rubbed in. The shoulder of the pivot rides on the jewel and controls the end shake of the arbor.
Below are the 5 most common types of jewels used in watches and where they are used.
This has a straight-sided hole and are used for general purposes in the train of wheels.
The curved or “Olive” shaped hole in this jewel is used to provide lower friction but have less wear resistance. Because of the shape they can handle a slight angular misalignment over a Cylindrical jewel hole. These are typically used at low torque areas of the train of wheels.
The Centere Wheel Jewel
This is a straight sided jewel hole with a larger diameter hole used for the high torque Center Wheel.
The Convex or Balance Jewel
Balance Jewels have a ‘Olive Hole” along with a curved stone. It is used for the upper jewel on the balance cock and lower Balance jewel on the main plate. They are also used for the pallet fork and escape wheel in higher quality watches.
The End stone Jewel
End stones utilize a flat lower surface with a domed top and are used along with the Balance Jewel. The flat surface is where the tip of the balance pivot rotates when the watch is in the horizontal positions.
The balance jewel and end stone together
Since the tip of the balance pivot rotates on the end stone in the horizontal position, (minimal friction) and the larger surface of the balance pivot rotates on the “Olive Hole” of the Balance Jewel ( maximum Friction) when in the Vertical positions. This increased friction is one of the main reasons you see a drop in amplitude between the horizontal and vertical positions.
The curved part of the jewel is called the oil sink. This is where the oil sits along with the pivot in is held in place with surface tension. It is especially important when oiling a Jewel hole to keep the oil in the oil sink. Too little oil starves the pivot of proper lubrication. Too much or sloppily placed oil, will migrate away from the pivot and jewel hole, transferring to places where oil does not need to be. This can cause parts to foul, effectively reducing the life of the service.