The Seiko Museum Ginza on why they are there: "The areas with which the shafts are in contact are required to withstand wear so as not to reduce their accuracies or fail."
What they are: "A synthetic ruby, sapphire, or other hard stone are used, which are durable enough to have a high friction resistance." The word to notice is synthetic: these are manufactured stones, chosen for hardness. The page makes no statement about their value.
Where they sit: around the shafts as hole stones, with cap stones above and below on the escape wheel and balance, and at the pallet forks and impulse pin "which wear easily".
On jewel counts. The museum states: "more stones are required in higher-class and more complicated mechanical watches although minimum 17 stones are good enough for functionality. For example, Grand Seiko High Beat uses 37 stones." Note that the page ties the count to two things at once: how complicated a movement is, and what class it is, so it does not support reading the number as a measure of one alone. ISO 1112 governs how the count may be marked and advertised.
Sources
Every statement above traces to one of these. Read on the date shown.
- Tier 1 The Seiko Museum Ginza, "Why are Stones Required in Watches?", Knowledge — Trivia, source read 2026-09-26