Under the Loupe/Jewelling
Today, the functional jewels used in watch and clockmaking are made from synthetic corundum, most often in its "ruby" state. These synthetic, typically flawless, stones are created thanks to a method developed by Verneuil in 1892. In practice, synthetic ruby has many great advantages. It is very resistant to wear due to its hardness (9 on the Mohs scale). Its coefficient of expansion is negligible under the effects of heat and cold. It is by no means magnetic, and it is very chemically stable.
Types of Jewels in a Timepiece
There exists a variety of different forms of functional jewels in a watch, depending on their place and function. Below is a brief, though not exhaustive, list.
- Bushing (also spelled Bouching, from the French bouchon)
- Cap jewels
- Pallet jewels
- Roller jewels
How They Work
Jewelling takes on a variety of different functions in a timepiece. In the case of jewelled bushings, the jewels (also referred to as rubies) must ensure the correct centering, end-shake, and proper turning of the pivots. In modern watches, these jewelled bushings are friction set into holes pre-drilled into the mainplate and bridges. For this reason, the outer rim of these jewels are not as highly polished as the functional, lubricated surfaces. The correct positioning of these holes, and the corresponding holes in the center of the jewels, is essential to the proper running of a watch. Opposing holes must allow the wheels to run parallel between them and must also facilitate the correct depthing of the gear teeth from one wheel to another.
The roller jewel must be highly polished to reduce to a minimum the effect of friction in transferring energy from the pallet fork to the balance wheel. Its correct size and shape must also be considered in order to maximize this transfer of energy.
The jewels used for the pallets of the escapement must be highly polished, its surfaces perfectly flat, and the angles of engagement with the escape wheel precisely engineered to maximize the transfer of energy to the balance wheel. The surface of these jewels should be specially treated to promote the retention of lubrication at the contact surfaces.