Jewel

A jewel (German Stein or Lagerstein) is a synthetic ruby or sapphire bearing inserted into a mechanical movement at points of friction and pivot rotation. Jewel count — 17, 21, 25, 31 — is one of the classical specifications of a mechanical watch and is occasionally printed on the dial or the movement.

What jewels do

A typical mechanical movement has rotating pivots that turn many times per day — the seconds wheel once per minute, the balance staff 28,800 times per hour. Steel pivots in steel bores would wear quickly. Synthetic ruby (aluminium oxide with chromium impurities, Mohs 9) is one of the few materials harder than steel. Ruby bearings let steel pivots run for decades without measurable wear — and hold the lubricant in place at the same time.

The same material — manufacturable as a synthetic crystal since 1902 by the Verneuil process — is also used for the watch crystal on premium watches; in jewels the ruby is shaped into small precision bearings instead of a flat disc.

Where jewels sit

A modern mechanical movement typically carries 17 to 31 jewels, distributed across:

• Gear-train pivots. Each wheel uses two jewels — an upper and a lower pivot bearing.
• Balance staff. Two jewels at each end, plus a cap stone in the shock setting and the impulse jewel on the roller.
• Pallet fork. Two ruby pallets and, where applicable, end stones at the bearings.
• Escape wheel. Pivot jewels.
• Date and complication trains. Add more.

A classical three-hand watch with no complications uses around 17 jewels. A chronograph adds 4 to 8 more depending on layout. Perpetual calendars and complicated movements can reach 50 and above.

Jewel count and quality

A higher jewel count does not automatically mean a better movement. Within a given price tier the count is mostly a function of complication. Compare:

• ETA 2824-2 (basic Swiss automatic): 25 jewels.
• Rolex calibre 3235 (modern Submariner / Datejust movement): 31 jewels.
• Patek Philippe calibre 240 PS (micro-rotor): 27 jewels.
• A. Lange & Söhne L901 (Lange 1): 53 jewels.

Marketing-driven inflated jewel counts — additional non-functional decorative jewels — were a 1960s and 1970s problem and largely do not exist in current production. Modern jewel counts are functional.

Notes from practice

At our atelier in Munich we inspect the jewels under loupe or microscope during every service. Damaged jewels — chipped, cracked, loose in their setting — are rare but possible; the typical cause is a hard impact. Aftermarket-replaced jewels in vintage service work can be a hint of non-original repair: original-spec rubies age uniformly, while out-of-period replacements differ slightly in colour and finish.