Grand complication pocket watch
A very hard red stone that is a type of corundum (aluminium oxide). Ruby is especially suited to making bearings (jewels) for the watch's different moving parts and the organs of the escapement, thereby reducing friction to a minimum.
Drilled rubies were used for the first time by Nicholas Facio de Duillier in 1704.
Watches today use synthetic rubies known as jewels. Drilled and polished, they are used as bearings for the different pivots to minimise friction and wear.
As a general rule, a simple mechanical watch, i.e. one that indicates hours, minutes and seconds, should have at least fifteen jewels at the points most exposed to friction.