(Nicked and edited from various sources): Mechanical watch movements are protected against wear by "jewels""”very hard synthetic sapphires or rubies"”that have been machined by drilling, chamfering (symmetrical bevelling), and polishing to allow them to serve as bearings for the gears. The average mechanical watch has about 15 jewels that support gear shafts and reduce friction. The jewels at the most sensitive points of watch mechanisms are set in tiny springs so that they also protect the watch against shocks. (Hence, shock-resistant watches.) Natural rubies, sapphires, and occasionally other jewels (amtheyst, aquamarine) were used in watches starting in 1704. Watchmakers switched to synthetic jewels when production methods made it much less expensive to make them than to mine them. Typically, the basic 7 jewels are part of the escapement and balance, including cap and hole jewels for both the top and the bottom of the balance wheel (total of 4), the two pallet jewels and the roller jewel. The next 8, making 15 jewels, are hole jewels for the fast moving part of the gear train. The next 2, making 17 jewels, are hole jewels on the center wheel. The next 2-4, making 19-21 jewels, are cap jewels on the escape wheel and the pallet fork. Some really high-grade or ultra-thin movements will add a few extra jewels to further protect against any wear, upping the total to 22-23. Automatic winding movements will add about 4-8 jewels to help most efficiently transfer the relatively small rotor forces into winding the mainspring.Â Also, chronograph and perpetual calendar movements may add to the total, as might more esoteric complications. About 50 years ago, some manufacturers, marketing to the "more-is-better" mentality, came up with 75 to 100 jewel movements. Of course, most of these jewels were not functional, and informed consumers knew better, so the trend didn't last long.