Just to add to what David has said, jewels were found to be useful as they are hard and allowed a watch to run a bit smoother and for longer. I belive the friction from a pivot running in a jewel is fractionaly less than a pivot running in a brass hole. As jewels are harder than brass they are also less prone to wear but they still need to be checked carefuly as they do wear. They also can become chipped and cracked. Quite often with cap jewels and endstones you will find little wear spots where the pivot end runs on them, if this is the case it is best to replace them. Because this is a more expensive process where the manufacturing of the watch was concerned you generaly find that the least expensive movements have no jewels and the top of the range movements from 15 upwards. This became a selling point, it was belived the more jewels the watches had the better the movement, companies picked up on this and started putting silly amounts of jewels in watches, most of which didnt actually do anything. It was a bit like how they sell digital cameras now, people think the more megapixels the better, that is not quite true unless you want photographs 10 meters square.
I hope this explains a little and please ask if you have any questions.
I need to learn how to remove and replace rubbed in jewels, I think Bob should cover this in a future video 
Paul.
