Jewels in watches are much more prosaic than their name suggests.
When we read about terms used in mechanical watchmaking, we often hear terms like “jewels” or “rubies,” and we often fantasize about the hidden riches inside our watches. There are some fiction stories where thieves steal the jewels of a watch!
Well, the term “ruby” or “jewel” is used to refer to tiny artificial stones (jewel bearings) that are set inside the watch. They are shaped like a torus, are man-made, and I must add, practically worthless – sorry for shattering your dreams of becoming rich in this way.
If you examine a watch movement, you can notice that these rubies are set in particular places (so, they are not ornamental, even if they do have a definite decor effect).
They are usually set in pairs (one above and one below) in correspondence of the pivots of the gears, and they are generally drilled through with a tiny hole.
You can see it here (it’s a cross-section) what it happens. The pivots of the gears are set precisely through these stones, one up, one down, so they can rotate freely.
And why using jewels instead of plain metal bearings? (mind you, lots of watches use bearings too – especially lower-cost mechanical watches and quartz mechanisms).
The answer is, because of attrition.
Attrition and jewels
First of all, a small note. Everybody knows that the hardest substance on Earth is the diamond. The hardness of materials has been measured by a scale, known as Mohs Scale. The diamond has a value of 10 on this hardness scale. On the other hand, the talc, with just a value of 1. Most of other materials stay between these two extremes. Well, corundum has a value of 9 on the Mohs scale – and rubies and sapphires are two forms of corundum. Metals have a lesser value by much: gold has 2.5, brass and nickel have 3, platinum has 4, iron and steel 4.5, titanium has 6, and tungsten carbide has 9.
You can see that a jewel is harder, even if more brittle than its metal counterpart, the bushing, so a steel pivot rotates inside it more effortlessly and with less grinding on it than it would in a bearing made of metal (usually, brass). The static coefficient of friction of brass-on-steel is 0.35, while the one of sapphire-on-steel is 0.10–0.15, so three times less.
It means a smoother and better transmission of movement, less attrition, and extended durability of the watch without constant maintenance.
As you can easily imagine, setting a jewel inside a micro-mechanics element like a bridge of a watch is a complicated issue. This means that jewel setting used to be reserved for higher-quality (and higher cost) watches.
Use of jewels in watchmaking
Jewel bearings were introduced in watches by Nicolas Fatio (or Facio) de Duillier and Pierre and Jacob Debaufre around 1702. Still, they did not become widely used as they were very costly. The first jewels were, indeed, shards of real gemstones. Watches often mounted garnet, quartz, or even glass; only the top quality ones mounted sapphire, ruby, and even diamond jewels.
In 1902, everything changed, because Auguste Verneuil developed a chemical process to create synthetic jewels. Hence, they became quite cheaper and gained widespread use in watchmaking. Jewels in modern watches are generally rubies or corundum, one of the hardest substances known (apart from diamond).
So, modern watches tend to use jewels on every part that is subject to constant grinding of metal against metal. This includes the pivots of the wheels of a typical wind-up watch (wheel train, escapement wheel, balance wheel), as well as two other critical elements: the pallet fork endings and the single impulse jewel in the center of the balance wheel.
Along the more ordinary torus-shaped jewels housing the pinions of the wheel train wheels, we also have some unique jewels called capstones. These jewels are used in wheels where friction is critical, as the balance wheel pinions. They are set so to prevent the shaft of the wheel from touching the surface of the jewel, and also, to create a space called “oil cup” which helps to lubricate the mechanism better.
More often than not, these capstones are held in place with shock-resisting mechanisms, such as the Incabloc.
Jewel-count. Beware of exaggerations!
Before the introduction of shock-resisting systems (around 1932), the best wind-up mechanical movements without complications mounted 15 jewels. After that, the jewels rose to 17. Automatic and complicated movements usually mount more jewels, as they have more wheels requiring them.
From their introduction, watch manufacturers defined the quality of the movements using terms such as “XX Jewels,” where XX was the number of jewels mounted into the watch.
This term diffused itself in product descriptions, becoming a natural equivalence to the public of “more jewels, more quality.”
So much that some companies, between which Waltham and Orient, launched on the market watches featuring 100-jewel movements. These movements used aesthetically-placed jewels, which had no function whatsoever, except to let the manufacturer say that there were 100 jewels inside the watch.
This example is a practical effect of the fascination that a name like “jewel” can have on someone who does not know the functional aspects of jewels inside a watch.
We should note that this practice, which was deceiving for the customers, was first condemned, and then prohibited by law.
Today, a manufacturer cannot place jewels inside a movement that have no practical purpose. If it does, he cannot refer to them in its communication.
You can find much more about horology and its fascinating history in The Watch Manual, a thorough e-book that explains all the basics about watchmaking and its protagonists.