Ruby

Screwed [[chaton]] at a movement of\\ [[A. Lange & Söhne]] with red **ruby** (click to enlarge!)
Screwed chaton at a movement of
A. Lange & Söhne with red ruby

A ruby, in the case of wristwatches, is usually a synthetic ruby. It is used in watch movements as a low-friction cover or bearing stone, in which the pivot of a wheel turns.

More see Jewels.