What is the "jewel" in the movement of a Rolex?

* photo of the Rolex Jewel


Defined, a "jewel" is a synthetic sapphire. Older watches and small productions still use genuine rubies but overall they are synthetic. The number of jewels vary from watch to watch for example a Rolex 3135 has 31 jewels and a Rolex 4131 has 41 jewels. The number of jewels is not a sole indicator of how good the movement is but it is a factor. Jewels do wear over time and need to be properly oiled/serviced. They act in some cases like bearings.

On a scale 1 -10 in hardness, where a diamond is a 10, a jewel is a 9/10 in hardness.

On the photo, you can see a small dish are in the center of the jewel. This is where the oil pools for lubrication of the movement.

Each jewel is very precise in size.