What does 21 jewels mean in automatic watch?
Jewels are used in watches at friction points. You can get them to a very fine point and they will not wear down like metal. So at a point that a gear will be turning for days and days, they use a jewel. if they used metal the point would soon wear to a round nub. Like the end of a ball point pen. increasing friction, and slowing the watches movement. Decreasing the accuracy of the watches timekeeping ability. Usually the higher amount of jewels in a watch the more accurate. Also more moving pieces. Historically the more jewels the more expensive. That was when all the watch pieces were made by hand, and needed to be very precise. Now that we have CNC and computers making very precise components, that old adage isn't so true. Jewels are used in all watches that have gears and movements in them. Not just Automatic watches. The newer watches that are basically small printed circuits and LED readouts don't have them, because of no moving parts. And I'm not sure if new cheap watches with moving hands, they may have little electric motors. But if it is old, or new and you can see moving gears when you remove the back. It has "Jewels".