What are Jewels in a Watch Movement

The jewels in a mechanical watch movement are synthetic rubies used as bearings at points where metal components pivot against each other. They reduce friction and wear, improving the movement's accuracy and longevity. Every mechanical watch contains jewels, typically between 17 and 31.

Why Rubies

Synthetic ruby (crystalline aluminum oxide, or corundum) has two properties that make it ideal for watch bearings.

First, it is extremely hard. Ruby scores 9 on the Mohs hardness scale, second only to diamond. This hardness means the bearing surface resists scratching and deformation under the repeated stress of a pivot rotating against it millions of times per year.

Second, ruby has a very low coefficient of friction against polished steel when lubricated with watch oil. A steel pivot rotating in a ruby bearing generates far less friction than the same pivot in a steel or brass bearing. Less friction means less energy wasted as heat, more energy reaching the balance wheel, and more consistent torque delivery throughout the power reserve.

Synthetic rubies have been used in watchmaking since the early 20th century. Before synthetic production methods were developed, natural rubies were used, which made jeweled movements significantly more expensive. Today, synthetic ruby is inexpensive to produce, and the cost of jewels in a movement is negligible compared to other components.

Where Jewels are Placed

Jewels serve as bearings at specific pivot points throughout the movement. Each pivot typically uses two jewels: one above and one below the gear wheel, forming a low-friction bearing pair. The pivot (a thin steel post extending from the wheel's arbor) sits inside a hole drilled through the center of each jewel.

The critical locations for jewels in a standard mechanical movement are:

Balance wheel pivots. The balance wheel oscillates thousands of times per hour, making its pivots the highest-wear point in the movement. These always have jewel bearings, typically with cap jewels (an additional flat jewel on top of the hole jewel) to limit vertical play. This bearing arrangement is called a shock-resistant setting and includes a spring-loaded mount that allows the jewel to absorb impacts without transmitting the shock to the delicate balance staff.

Escape wheel pivots. The escape wheel rotates rapidly and interacts with the pallet fork at every beat. Its pivots require jewels to maintain consistent friction.

Pallet stones. The two stones on the pallet fork that engage with the escape wheel teeth are jewels (typically rubies, though sometimes garnets). These are not bearings but functional surfaces that lock and release the escape wheel. Their hardness ensures that the locking and impulse faces do not wear and alter the geometry of the escapement over time.

Impulse jewel. A small jewel (sometimes called the impulse pin) mounted on the balance wheel that interfaces with the pallet fork. It transmits impulse from the escapement to the balance wheel.

Gear train pivots. The center wheel, third wheel, and fourth wheel pivots may have jewel bearings, depending on the quality level of the movement. In movements with 17 jewels, only the most critical pivots (balance, escapement, and pallet) are jeweled. In 21-jewel movements, the gear train pivots receive jewels as well.

Automatic winding mechanism. In automatic movements, the rotor bearing and the pivots of the reverser wheels may have additional jewels. This accounts for the higher jewel counts (25 to 31) found in automatic calibers compared to manual wind calibers (typically 17 to 21).

How Many Jewels Does a Movement Need

The minimum jewel count for a functional mechanical movement with reasonable accuracy and longevity is 7, covering only the escapement and balance wheel. However, a 7-jewel movement will wear faster and lose accuracy sooner than a properly jeweled one.

The practical minimum for a quality manual wind movement is 17 jewels: 2 for the balance staff, 2 cap jewels for the balance, 2 pallet stones, 1 impulse jewel, 2 for the escape wheel, and 2 each for the center, third, and fourth wheel pivots.

For automatic movements, 21 jewels is a common standard, adding bearings for the automatic winding components. Higher-end automatics with 25 to 31 jewels include jewels at additional secondary pivot points, the rotor bearing, and sometimes decorative jewels on the automatic bridge.

Jewel Count and Quality

A higher jewel count does not necessarily indicate a higher quality movement. This is worth emphasizing because jewel count was historically used as a marketing tool.

During the mid-20th century, some manufacturers added jewels to non-functional locations (such as under screws or on stationary plates) purely to inflate the jewel count and imply higher quality. A movement advertised as having 51 or 75 jewels almost certainly contains non-functional jewels that serve no horological purpose.

In 1974, the International Organization for Standardization published ISO 1112, which defined rules for jewel count marketing. Under this standard, only functional jewels (those that serve as bearings or working surfaces) may be counted in the advertised jewel count. This largely ended the practice of jewel stuffing, though some manufacturers still occasionally produce movements with unnecessary jewels as a novelty.

A 17-jewel ETA 6497 and a 36-jewel Patek Philippe 240 are both properly jeweled movements, each with jewels at every point where they serve a functional purpose. The Patek has more jewels because it has more components (micro-rotor, additional complications), not because it is proportionally better jeweled.

Care and Service

Jewels themselves do not wear out under normal conditions. They are harder than the steel pivots that rotate within them and will outlast most other components of the movement. What does degrade is the lubricant applied to the jewel bearings. Watch oils break down over time, dry out, or migrate away from the bearing surface. When this happens, friction increases despite the jewels being present, and the movement's accuracy and power reserve decline.

This is the primary reason mechanical watches require periodic servicing. The jewels are cleaned, the old lubricant is removed, and fresh oil is applied to each bearing surface. The jewels themselves are inspected for cracks (which can occur from impacts) but are rarely replaced unless damaged.