Jewels have two important properties that help reduce friction. First, they can be made to be very smooth, and therefore they let the metal parts slide easily. Secondly, they are very hard and therefore don't wear down very quickly. The gears in a watch are carefully designed so that the teeth roll on each other, rather than sliding. If the axle of a gear wears away the hole that it sit in, the gear will shift. That means the teeth will no longer roll on each other and therefore friction will be increased.
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