The majority of stones (jewels) used in watches today are synthetic. Their role is to minimise friction.
A precious stone is a hard and rare crystalline mineral, used in jewellery and for certain parts of precision instruments such as watches, clocks, and compasses. The four precious stones are the diamond, ruby, sapphire (both corundums) and emerald (beryl).
A fine stone is a mineral having the same properties as a precious stone though to a lesser degree. The main fine stones are topaz, spinel, tourmaline, garnet and quartz (rock crystal, agate, amethyst).