English: Drawing of a jewel bearing and capstone in a mechanical watch. The bearing is sectioned through the axis. The jewels (red) are made of synthetic ruby. The lower jewel is called the 'hole jewel', the upper one is the 'capstone' or 'end jewel'. This type of bearing is used in watches where friction is critical, such as in the balance wheel pivots. With ordinary bearings (shown in Watch jewel bearing.svg), when the watch is vertical the shoulder of the shaft bears against the face of the hole jewel, increasing friction. This causes the watch's rate to vary with its position. In contrast, in this type, the capstone provides a low friction surface for the rounded end of the pivot to press against instead, reducing friction. The hole for the pivot is slightly convex (hourglass shaped) so that if the pivot is not exactly vertical it won't jam in the hole. The surface of the lower jewel ('bombé') is also slightly convex, holding a drop of oil (yellow) between the jewels, in contact with the pivot, by capillary attraction. The jewels are press-fitted into holes in the movement's supporting plates (grey). 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