Section 13: Watch Jewels — Everything You Need to Know About Jewels in a Watch Movement

In 1704, natural diamonds were first used as bearings in watch movements — and patented. As pivot bearings, natural diamonds dramatically improved the transmission efficiency of gear trains, greatly increased timekeeping precision, and significantly extended the service life of the movement.

At the time, the use of diamonds in watches was a closely guarded trade secret. Many manufacturers subsequently attempted to replicate the technique, but the selection, drilling, and installation of natural diamonds proved extremely difficult and costly.

In 1892, the French invented synthetic jewels. Their primary component is aluminum oxide (Al₂O₃), sintered at high temperature. Red, blue, white, and other colors are produced by adding different colorants during the sintering process. When chromium oxide is used as the colorant, the result is the familiar red jewel commonly seen in watch movements.

Every industry has its conventions, and the watch industry's convention regarding "jewels" is precise: "jewel" refers to synthetic jewels, not natural diamonds. If a natural diamond is used in a specific position, this must be explicitly stated.

The jewel count of a watch is one of the defining characteristics of its specification. Before the advent of quartz watches, the number of jewels marked on a watch was an important indicator of its quality. Explore fine mechanical timepieces at Aorawa Time.

I. The Eight Types of Watch Jewels

The jewels used in watch movements can be classified into approximately eight types based on their structural form:

• (a) & (b) Straight entry and exit pallet stones — used in the pallet fork (lever escapement)
• (c) Impulse pin (roller jewel) — the circular disc jewel mounted on the impulse roller
• (d) Spherical cap jewel — used as a shock jewel on the balance staff, convex dome shape
• (e) Spherical arc hole jewel — used as a shock jewel, with a concave spherical seat
• (f) Flat straight hole jewel — used for the barrel wheel, center wheel, third wheel, and fourth wheel
• (g) Grooveless straight hole jewel — used for the barrel cover, center wheel, etc.
• (h) Grooveless straight hole jewel (chamfered opening) — used for the barrel cover, center wheel, etc., with a chamfered entry

II. The Key Properties of Watch Jewels

Synthetic watch jewels possess four outstanding properties that make them ideal for use as movement bearings:

1. High hardness — Mohs hardness 9, second only to natural diamond (Mohs 10)
2. Excellent wear resistance
3. Good surface roughness — capable of achieving an extremely smooth bearing surface
4. Low coefficient of friction

Experiments have demonstrated that when steel components and watch jewels are used together under dry friction conditions, the friction coefficient is approximately 0.14. Under good lubrication conditions, this can be reduced to as low as 0.01.

III. The Distribution of Jewels in a Watch Movement

In a standard hand-wound base movement, 17 jewels are sufficient. They are distributed as follows:

• Center wheel: upper and lower jewels (2 jewels)
• Third wheel: upper and lower jewels (2 jewels)
• Fourth wheel (seconds wheel): upper and lower jewels (2 jewels)
• Escape wheel: upper and lower jewels (2 jewels)
• Pallet fork: upper and lower jewels (2 jewels)
• Balance staff: upper shock jewel (1 jewel) + lower shock jewel (1 jewel) = 2 jewels
• Pallet fork: entry stone, exit stone, and impulse pin (3 jewels)

Notes:

• A 19-jewel movement adds one jewel above and below the barrel wheel.
• A 25-jewel movement adds a further 6 jewels for the automatic winding mechanism.
• In watchmaking convention, the side facing the caseback is called "upper" (top); the side facing the dial is called "lower" (bottom). The movement surface viewed from the caseback is the assembly side; viewed from the dial side, it is the dial side.

IV. Functional Jewels vs. Non-Functional Jewels

Jewels that directly reduce friction in the movement are called functional jewels. Those that do not directly reduce friction are called non-functional jewels (decorative jewels).

Some manufacturers have, in the past, marked non-functional (decorative) jewels on the dial to inflate the stated jewel count — watches with 88 jewels have appeared on the market. This practice is clearly misleading. Accordingly, ISO standard 1112 stipulates: only functional jewels may be marked on the dial; non-functional jewels, regardless of quantity, must not be indicated.

The jewels inside a mechanical watch are not decoration — they are precision engineering, measured in microns and hardness grades. Understanding them is understanding what separates a fine movement from an ordinary one. Explore timepieces built with precision at Aorawa Time.