Watch jewels are not decorative. They are tiny bearings, typically synthetic ruby or sapphire (both forms of corundum, Mohs hardness 9), pierced with a precise hole through which the pivot of a wheel spins. Corundum's hardness and low coefficient of friction against steel make it an ideal bearing material: it does not wear significantly over decades, it holds lubricant well, and it does not corrode. The first watch jewels were natural ruby, garnet, or diamond; modern jewels are synthetic, grown from alumina since the early 20th century.
The watch jewel was patented by Swiss mathematician Nicolas Fatio de Duillier in London in 1704. Fatio developed the technique of piercing the small stones and fixing them into the movement's plates and bridges. For a century the patent was an English and then Swiss industrial secret, and jewelled watches were the top of the market. From roughly the 1840s onward, pierced-jewel technology spread across the industry; today every serious watch movement from every manufacturer is fully jewelled.
The jewel count marketing battle of the 1960s is largely a historical curiosity today. A minimum of 17 jewels was historically required for COSC chronometer certification (though the current standard does not specify a count). A typical automatic has 25 jewels: the two pivots of each of the four train wheels (8), balance wheel (2 pivots + 2 impulse + 1 roller = 5), pallet lever (2 + 2 = 4), escape wheel (2), plus the automatic-winding system's additional bearings. Complicated movements add more: Rolex Cal. 4130 has 44 jewels; the Patek minute repeater Cal. R 27 PS has 39 jewels; the Jaeger-LeCoultre Cal. 947 Westminster has 123.
There is no direct correlation between jewel count and movement quality past the 25 to 31 range. Additional jewels above that number typically reflect additional complications or elaborate engineering (extra bearings in a chronograph module, for example), not fundamentally better performance. A well-finished ETA 2824 with 25 jewels will be as accurate as an over-jewelled competitor. Jewel count does, however, remain a useful proxy for movement complication: a 40+ jewel watch almost certainly has a chronograph, annual or perpetual calendar, GMT, or other serious mechanism.
