Definition
A small, hard, highly polished bearing made from synthetic sapphire or ruby, used to support the pivot of a delicate moving shaft in precision aircraft instruments. The hardness and smoothness of the jewel reduce friction and wear, allowing pointers and balance arms to move freely in response to very small forces.
Plain English
A tiny gemstone-like bearing inside an instrument that lets a fine shaft spin or pivot with almost no friction, so the instrument can respond to small movements accurately.
Context Anchor
Seen in discussions of aircraft instrument construction, instrument maintenance, and instrument faults such as sticking or sluggish pointer movement.
Derivation
The word jewel comes from the Old French jouel, meaning a small precious object. Synthetic sapphires and rubies have been used in watchmaking and instrument-making for over a century because they are extremely hard and can be polished to an almost perfectly smooth surface, which is why the bearing carries the name.
Why Pilots Care
Jewel bearings keep instrument needles and mechanisms moving smoothly and accurately for hundreds of hours without binding or excessive wear.
Analogy
Like the tiny ruby pivots inside a fine wristwatch -- they let delicate parts turn with almost no resistance.
Intuition Check
Do not think of “jewel” as decoration here. In this term, the jewel is a hard, smooth working surface used to reduce friction in a delicate instrument.
Example Sentence 1
The shop sent the airspeed indicator out for overhaul after a jewel bearing was found chipped during inspection.
Example Sentence 2
During overhaul the mechanic checked each jewel bearing for scoring before reassembling the altimeter.