There are two types of jewels that are used in watches: functional and non-functional.
Non-functional perform a decorative role.
Functional jewels are used for stabilizing the friction and reducing the degree of wear of the contacting surfaces of the mechanism. It is designated as a function of the jewels in NIHS 94-10 standard, adopted in 1965 by the Swiss organization NIHS (Normes de l'industrie Horloge Suisse).
Why exactly precious stones? The jewels do not corrode, and grinding a stone, it is possible to obtain a perfect and long-term clean surface; among their advantages there are good temperature stability, and the ability to operate without lubrication and in corrosive environments. That is why watchmakers use jewels instead of other cheaper materials such as brass.
In addition to pillar plate, the jewels are used in two other places, which are under intense attack. Pallets that are fastened on the shoulders of the pallet fork and the pulse stone are made of jewels too. Only a very strong mineral can withstand the pressure of the anchor tooth wheel and blows of the horns of the pallet fork, so synthetic ruby is used. For its processing watchmakers use diamond tools.
Not surprisingly, that watch jewels have become a real boon for watchmakers in the XVIII century, when started the pocket watch era. The mechanisms have become so small that the parts under the clockwork spring pressure were quickly coming into disrepair.
The first watch with jewels in the mechanism were issued in 1704. But the idea of using jewels in so unusual way is belonged to the great English clockmaker George Graham (1673-1751)
Watch jewels are divided into four groups: