Sometimes you see that a mechanical watch has 17 Jewels, or 13 Jewels, but what does that mean? They are called Jewel Bearings and are placed at the center of a rotating gear wheel. The jewels are  synthetic Rubies and their purpose is to minimize friction between hardened steel parts, increase accuracy, lower the weight of the movement and  help lubricate the movement with a very thin layer of special oil.

Theoretically, more jewels  means better precision, but sometimes watch makers add non-functional or unnecessary jewels to give the impression of accuracy. Usually, a fully jeweled timpepiece has  around 17 jewels.

The Jewel Bearings were invented in 1704 and, at the beginning, natural  stones  were used, such as sapphire, ruby and gamet. Almost two centuries later a process to make synthetic  rubies was invented and since then they are used, mainly because of its lower costs.

Jewel Bearings in a automatic watch

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