Made of synthetic ruby grown in a lab, jewels in watch movements are used to reduce friction in a movement. Invented in the 18th century, they function as bearings for metal pivots of crucial components, for instance the balance staff and pivots of the gear train.
These days, the number of jewels in a watch movement is no longer an indicator of high quality, as they used to be a long time ago, when synthetic rubies were harder to come by.
Nonetheless, one brand that has made an art form out of the use of jewels is A. Lange & Sohne, whose distinctive movements stand out with their combination of rubies, gold chatons (circular pieces of metal into which the rubies are set), and blued steel screws. Additionally, the watch shown here, the Datograph Perpetual Tourbillon, features a diamond endstone, a signature of A. Lange & Sohne tourbillon watches.