I’m not talking about iced-out watches with diamonds on the dials and straps, but take a simple Rolex, for example. I watched a restoration video recently, and I couldn’t for the life of me figure out why there were several tiny gemstones inside, along with all the gears and the springs. What mechanical purpose do these stones serve?
The jewels act as bearings for the gears in a mechanical watch.
Without a jewel, the metal axle of each gear would be sitting directly on the metal body of the watch movement, and even lubricated this metal on metal contact would result in more friction. The more friction there is, the less efficiently the mechanism will run, the quicker it will wear out, and the less accurate it will be.
With a large device we would support the axle on bearings to lower this friction and allow it to move more smoothly. Great in theory, but when it comes to a watch it just isn’t really possible to make standard bearings small enough. Instead we use jewels.
A jewel is a small artificial ruby, which is much harder and smoother than metal, so it allows the gears to spin with much less friction and wear.
Originally these jewels were expensive and hard to manufacture, so they were only used for the biggest and most critical pivot points in the movement. Over time they were then used at more points throughout a.movement to help make the movement that little bit more efficient again, and also as an advertising point because more is obviously better.
The jewels are rubies or sapphires (or a synthetic version of them), and they act as a bearing surface for the moving parts. The aluminum oxide the jewels are made of is a very hard surface but can be polished to have very low friction, so it doesn’t degrade or cause wear and friction to the watch movement.