Ruby is technically known corundum, and is a crystallized form of aluminum oxide (Al2O3). In pure form, corundum is white in color; trace impurities are added to change the color - to red in the case of rubies. It should also be noted that any other color of corundum (including clear) is known as sapphire.

Ruby is used because it is an extremely hard and provides a slick surface for the wheel pivots (and other steel components) to operate on. In a mechanical watch, there is a constant force applied to the pivot of every wheel in the wheel train, which is applied by the wound-up mainspring.

Without any jewels, the steel wheel pivots would very quickly grind away the bridge and plate material until the wheels came out of alignment, and the movement would crash to a halt. In the inexpensive watch of yesteryear, the pivot holes may have been provided with hardened metal bushings.

Ruby is significantly better than steel in handling the forces involved, wearing long, and providing a nice low friction surface suitable for both high-load as well as high-speed motion.

(used with permission of TimeZone)