What are jewels?
Watches have many moving parts, with wheels and pivots and shafts that can cause wear where they are held in place. So instead of metal moving against metal, which would cause friction and wear, watch makers designed jewels which cause very little friction or wear. The jewels can be made of diamonds, sapphires, or most commonly rubies, with a small hole drilled in them to hold the shaft of a pivot or wheel.
More jewels are an indicator of watch quality, in general the higher the jewel count, the less wear and friction the watch will have, and the more accurate and long lasting the timepiece will be.
There are many different kinds of jewels including balance, friction, flat, center, pallet and roller jewels. Each are physically different, but are designed to reduce friction and wear on a moving metal part inside a mechanical watch. Each jewel will help the watch run more smoothly, more accurately and have a longer life. Each jewel adds to the cost and complexity of a watch movement, therefore a watch with more jewels costs more to make and makes the watch more valuable and long lasting.