Buy the first automatic watch, when looking at a case back, people often ask what are the little red dots on their watch’s movement or why watches all mention the number of jewels?
Jewels (rubies) are used as bearings to reduce friction at critical points, to improve the accuracy and durability of watches, because of a mechanical movement has made by metal to metal such as gears, wheels and pinions. These parts are rotating and it's the same level material if rotations every day it will slow the movement of the wheels down – ruining the accuracy of the watch.
Jewel bearings are a good solution, due to Jewel (Rubies) is one of the hardest materials after diamond, As a consequence, jewels can be made very smooth which allow low friction with metal pivots. Last, they have very good temperature stability and are acid-resistant. In some cases ruby (corundum). The jewels you can see in watch movements are usually red/pink in colour.
Do you know: Early age watch used Jewell originally natural ruby but now use synthetic ruby