Jewels in a watch are tiny synthetic rubies used as bearings at the points where the movement's gears turn. They aren't there for decoration or value — they reduce friction and wear where metal parts would otherwise grind against each other.
What the jewels actually do
Inside a mechanical movement, gears spin on thin metal pivots thousands of times an hour. Run metal on metal and it wears out. Hard, smooth synthetic ruby is nearly frictionless and barely wears, so watchmakers set jewels at the high-stress pivot points and use them to hold lubricant in place. Modern jewels are lab-grown corundum, not mined gemstones, so they cost little and stay consistent.
What “17 jewels” means
You'll see a jewel count printed on movements or in the specs — 17, 21, 25, and so on. A basic hand-wound movement needs around 17 jewels to cover the essential bearings. Add complications like automatic winding or a date, and you need more jewels for the extra moving parts. So a higher count usually means a more complex movement — not a better or more valuable one. Adding jewels beyond what the mechanism needs does nothing.
Does the count matter when buying?
Not much on its own. Don't shop by jewel count. It's a rough hint at complexity, not quality. A well-made 17-jewel movement beats a poorly made 25-jewel one every time. And quartz watches have few or no jewels, simply because they have far fewer moving parts.