Jewels are hard bearing surfaces — historically garnet, ruby or sapphire, later synthetic corundum — set where pivots turn, where the balance receives end-shake, and where the pallet stones and impulse jewel transmit impulse.
A seven-jewel watch typically jewels the balance and escapement only. Fifteen jewels extends jewelling through the train. Higher counts add jewelled barrel arbors, centre wheel bearings, or additional cap jewels — real improvements, but with diminishing returns.
This is why counts above roughly twenty-one on a vintage movement deserve scrutiny. Some are genuine high-grade work; others reflect jewels placed where they do no mechanical work, added for marketing or for tariff and import classification reasons.
Read the count alongside the adjustment statement. 'Adjusted to five positions, temperature and isochronism' describes labour spent on timekeeping; a bare jewel count does not.
Verify by inspection where possible. Jewel settings are visible, and a stated count that the plates cannot support is a sign of a replaced or misdescribed movement.