English: Drawing of jewel bearing used in a mechanical watch. The donut-shaped bearing is shown sectioned through its axis. This type is called a 'hole jewel', used for most of the ordinary wheels in the gear train. The bearing (red) is usually made of synthetic sapphire (ruby). It is press-fitted into a hole in the movement's supporting plate (grey). The cup-shaped depression in the top of the jewel is the oil cup; it's purpose is to hold the lubricating oil (yellow) in contact with the bearing shaft by capillary action. In wheels where friction is critical, a 'capstone' (shown in Watch jewel bearing and capstone.svg) is added on the end to prevent the shoulder of the shaft from bearing against the face of the jewel. Information for this drawing came from Henry B. Fried (1954) Bench Practices for Watch and Clockmaker, Arlington Book Co., Virginia, USA, Book 3, Ch. 1, p.140-188