It is doubtful whether adding jewels in addition to the ones listed above is really useful in a watch.
It does not increase accuracy, since the only wheels which have an effect on the balance wheel, those in the going train
, are already jeweled. Marine chronometers
, the most accurate portable timepieces, often have only 7 jewels. Nor does jeweling additional wheel bearings increase the useful life of the movement; as mentioned above most of the other wheels do not get enough wear to need them.
However, by the early 20th century watch movements had been standardized to the point that there was little difference between their mechanisms, besides quality of workmanship. So watch manufacturers made the number of jewels, one of the few metrics differentiating quality watches, a major advertising point, listing it prominently on the watch's face. Consumers, with little else to go on, learned to equate more jewels with more quality in a watch. Although initially this was a good measure of quality, it gave manufacturers an incentive to increase the jewel count.
Around the 1960s this 'jewel craze' reached ridiculous heights, and manufacturers made watches with 41, 53, 75, or even 100 jewels.
Most of these additional jewels were totally nonfunctional; they never contacted moving parts, and were included just to increase the jewel count. For example the Waltham 100 jewel watch consisted of an ordinary 17 jewel movement, with 83 tiny pieces of ruby mounted around the automatic winding rotor.
In 1974, the International Organization for Standardization
(ISO) in collaboration with the Swiss watch industry standards organization NIHS: Normes de l'Industrie Horlogère Suisse
published a standard, ISO 1112, which prohibited manufacturers from including such nonfunctional jewels in the jewel counts in advertising and sales literature.
This put a stop to the use of totally nonfunctional jewels. However, some experts say manufacturers have continued to inflate the jewel count of their watches by 'upjeweling'; adding functional jeweled bearings to wheels that do not really need them, exploiting loopholes in ISO 1112.
Examples given include adding capstones to third and fourth wheel bearings, jeweling minute wheel bearings, and automatic winding ratchet pawls. Arguably none of these additions adds to the accuracy or longevity of the watch.