When you are buying a fine timepiece with a mechanical movement inside, the technical specifications for the caliber usually list the number of jewels. Typically, those jewels are synthetic rubies, but sometimes brands use synthetic blue sapphires, as well — for a bit of a different look. The thing is, unless the movement is visible via a transparent caseback, you don't even see those jewels. So then, why use them at all? The reason is simple and effective: using synthetic jewels in the movement as bearings actually reduces friction within the caliber and therefore gives the movement a longer life by reducing wear and tear.
Adding jewels instead of mechanical metal pieces to do the bearings' job helps ensure accuracy. It also enables the watch brand to make the movement smaller in size and in weight than it would if the parts were made of metal. Additionally, rubies can withstand temperature changes and so offer stability. However, setting these minuscule jewels into their designated spots on the movement is no easy task. In fact, seasoned watchmakers must do this job using tweezers and microscopes. In the end, though, the look is beautiful, and it is great if you get to see the rubies (or sapphires) in all their glory in the timepiece.
As noted, these jewels are synthetically developed. Most use aluminum and chromium oxide that undergo heating, fusing and crystallizing processes. These rubies are not as valuable as natural rubies. The number of rubies that are used in each watch movement varies depending on the timepiece and its complexity. Typically, a three-hands watch will have about 11 to 17 rubies in it. Generally, though, more complex calibers and more moving components will demand the use of more rubies.