[Tishabet]

Originally Posted by Dixie

Please explain what the jewels actually do? I've always been curious.

Originally Posted by jrj3rd

Incredibly durable bearing surface. Prevents wear with a minimum amout of very light oil.

Yep, synthetic ruby is used as a sort of durable bearing surface in watch movements.

When you think of the "gears" in a clock or watch these will be made of metal and each one has a sort of axle, each gear needs to be precisely positioned relative to the next so that they can mesh perfectly. To hold the "axle" securely the synthetic ruby is used since it can be made to a very precise tolerance (so that axle isn't rattling around) but is also extremely durable and makes a low friction bearing surface. The "jewel" has a hole in the center to capture this "axle" and hold everything in place. (side note: in watch parlance the gears are called "wheels" and the axles are called "pivots" just to make things more confusing).



In the image below of the watch movement I was working on you can see a jewel pretty much dead center, this is the jewel for the pivot (axle) of the aptly named "center wheel" which you can see peeking out directly below it. If you look hard you can actually see the hole in the center of the jewel and the bit of "axle" visible there. So basically anywhere you see a purple jewel on a watch movement there is something rotating directly below it.



There are actually a few uses for jewels in a watch movement which are different than what I am describing above (in the balance and pallet fork) but that is more of a footnote... mostly jewels are used as described above. Last but not least, generally speaking super cheap watch movements have few or even no jewels (they use a metal to metal solution instead) so a high number of jewels became synonymous with "good watch." Broadly speaking this is true since a highly precise and highly durable watch movement will certainly be using jewels, but it led to some makers adding extraneous jewels for no purpose beyond saying they have more jewels than the other guy.