When it comes to mechanical watches, you often hear about jewels, stones, or rubies, which serve as so-called bearing jewels in every movement – whether from Rolex, Omega, or Chopard. On many watches, the number of jewels is stated on the dial, on the caseback, or – if the caseback is transparent – on the movement itself. This article explores what the number of jewels tells you about a wristwatch and its quality.
Precious Stones Ensure Smooth Operation
Even though they are often simply called stones for convenience, the stones used in watches are fundamentally gemstones. Watchmakers use them to make bearings, which is why they are also known as bearing jewels. On watches, the relevant markings often use the French terms Pierres or Rubis, or the English term Jewels. The French word Rubis means rubies, which also points to the fact that ruby is the most commonly used jewel bearing.
Depending on their shape and function, the jewels in a wristwatch are also called cap jewels, hole jewels, pallet jewels, ball-bearing jewels, or impulse jewels (roller jewels).
There is a specific reason why the bearings in watches are not made of metal but instead use jewels of ruby or other gemstones: this significantly reduces friction and wear. That benefits both the precision with which the movement runs and its longevity.
From Natural Gemstones to Synthetic Ruby Jewels
Gemstones were prized for their fascinating colors and brilliance as far back as pre-classical cultures, and to this day they are popular for all kinds of jewelry and goldsmith work.
From a physical standpoint, however, their main shared characteristic is not their luster or color but their exceptional hardness.
Diamond is considered the hardest gemstone, closely followed by corundum.
Watch jewels are frequently made of corundum; in the past, natural stones were used, whereas today mostly synthetic corundum is employed. The term corundum comes from Tamil and refers to a mineral from the oxide and hydroxide class that occurs relatively frequently in nature. There are various varieties that differ in color. These include colorless leucosapphires; deep red rubies, which owe their color to their chromium content; and sapphires, which occur in every color except red – although blue sapphires are the best known and especially popular as gemstones.
Although other types of corundum are suitable and are indeed used in practice, the best-known and most common jewel bearing is the ruby.
Since the late 19th century, it has been possible to produce corundum synthetically. Several processes are available, but the so-called electric arc fusion process is by far the most economically significant. It goes back to Ernst Moyat, a German chemist. In 1894, he became the first to produce what is known as fused alumina, and shortly before the outbreak of World War I he was granted a patent for his process. Over the following decades, industry developed high-grade fused alumina, of which mainly white fused alumina is produced today.
The reasons for the growing use of synthetic corundum in watchmaking were the sharply rising demand for high-quality watches and the fact that synthetic corundum met the material requirements – particularly in terms of strength and workability – just as well as its natural counterpart.
How Are Jewels Fitted in a Wristwatch?
The jewels used in watches are either pressed into a movement plate or screwed into a gold setting – also known as a chaton – and then adjusted. Because of their hole, some are also called hole jewels, and together with the metal pivots of the movement's wheels they form plain bearings. If a hole jewel without a cap jewel has a hemispherical recess, this is called an oil sink and serves to hold the watch oil used as a lubricant.
Jewels for watch movements were developed as early as 1704 by Nicolas Fatio de Duillier, a Swiss mathematician. In addition, there are also jewels in rolling-element bearings, for example in the ball bearings of automatic watch rotors. These, however, are cylindrical or spherical and have no hole. The term bearing jewels is not used for these stones. Another area where watch jewels are used is the escapement, where they serve as pallet jewels or as the impulse jewel. This solution was developed by Thomas Mudge, who is credited with inventing the lever escapement around 1757.
How Many Jewels Does a Wristwatch Need?
High-quality hand-wound mechanical wristwatches require at least fifteen jewels: ten hole jewels, two cap jewels for the balance, one impulse jewel (roller jewel), and two pallet jewels for the lever.
As a rule, the more complex the movement's design, the greater the number of jewels required.
The number of jewels, however, is not an indicator of a movement's quality; at most, it suggests the presence of complications. Nevertheless, many makers of luxury watches state that their watches contain 30 to 70 jewels. Here, though, reputation is the main factor, which also explains why the jewel count is often shown on watch dials and casebacks.
