Jewels act like the roller bearings or bushings of a watch's internals which are used to combat friction. Friction not only reduces the lifespan of the parts rubbing against each other, it affects accuracy as well. Since jewels were intended to be used on high wear points it made sense that the harder & smoother the material that they are made of, the better the reduction of friction. In 1704 the Swiss astronomer and optician Nicolas Fatio de Duillier patented a way of drilling rubies so they could be used in watch movements. The French company De Beaufré used his technique to manufacture the first watch that incorprated rubies into its mechanism. De Beaufré pocket watch from 1720: These gems were chosen because they consist of one of the hardest elements known to man and can be polished to an extremely sleek finish. They are also not adversely affected by temperature changes which would make other materials expand or contract and therefore alter tolerances. An added bonus to hardness is a longer lifespan. Both rubies and sapphires are the same element: corundum (crystalline aluminum oxide / Al2O3). Originally they were only naturally occurring and had to be mined, then cut with a diamond powder-coated lathe. Jewels also varied in quality. Lower to mid priced movements even used "seconds" - gems that would not pass visual inspection in regular jewelry because they had too many flaws. High end movements have top tier quality jewels mounted in "chatons" (gold cups) that are secured to the metal baseplate of the movement and then burnished. Lower level movement makers simply drilled the hole and attached the jewel directly to the metal. 1966 Longines caliber 370 with jewels in chatons: Interestingly enough, it's the impurities that give rubies and sapphires their color. The higher the content of other elements, the deeper the color. Chromium makes rubies red, and vanadium will give them a purple hue. Titanium and iron give sapphire a blue tint - the higher the iron content, the more yellowish green the color. Pure corundum is clear. In 1892 Auguste Verneuil discovered a process to manufacture synthetic gemstones. He published a paper on the making of synthetic rubies in 1902 which noted that the imitation substance surpassed it's naturally occurring counterpart for industrial uses. Besides being less expensive, the man-made version was practically free from defects and was of a greater purity. Before rubies could be synthesized, only the most costly watches had more than 7 jewels. After that time, it became a marketing game of who had the most since the general public was not aware of the lower price of synthetic gems. Waltham once made a 100 jewel watch! 83 of the rubies were totally useless; the other 17 were in standard functional positions. rotor of the 100j Waltham: photo by Paul Delury There were no regulations on jewels until 1965 when the NIHS (Normes de l'Industrie Horlogie Suisse) stepped in to police the advertising. In 1974 they ruled that all jewels should be rubies. Up until this standard was enacted there were many different grades of materials used (even glass!). The NIHS also defined what functioning and non-functioning jewels are in ISO 1112. The former is a "jewel which serves to stabilize friction and to reduce the wear rate of contacting surfaces of the components of a timekeeping instrument". Of course the latter is any jewel not used for this purpose. As with most laws there are loopholes. Some manufacturers add jewels where they do have a tiny function but are practically useless and don't improve accuracy, reduce much friction or add to the life expectancy of the unit.