Watch jewels, which help lubricate a watch's movement, are made of synthethic sapphires, such as rubies.
Natural jewels, including diamonds, sapphires, and rubies, were used from 1704-1902 before synthetic jewels were invented by Auguste Verneuil, a French chemist. This made jeweled bearings much cheaper and lowered the overall production costs of watch movements. Today most jeweled bearings are synthetic ruby or sapphire.
This extremely durable and resistant material--known as crystalline aluminium oxide or corundum--is also used in higher-end synthetic sapphire watch crystals that cover the face or dial of the watch because of its scratch-resistant properties.