mineral

Synthetic Ruby (Jewel Bearings)

Synthetic Corundum (Al2O3 doped with Chromium)

Hardness: 9 on the Mohs scale. Color: Deep red/pink. Luster: Vitreous/Adamantine. Crystal structure: Trigonal/Hexagonal. Cleavage: None. Specific gravity: 3.95-4.10.

Hardness: 9 on the Mohs scale
Color: Deep red/pink
Luster: Vitreous/Adamantine

Identified February 23, 2026More mineral →

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Physical properties

Hardness: 9 on the Mohs scale. Color: Deep red/pink. Luster: Vitreous/Adamantine. Crystal structure: Trigonal/Hexagonal. Cleavage: None. Specific gravity: 3.95-4.10.

Formation & geological history

Created via the Verneuil process (flame fusion) or Czochralski process. In horology, these are lab-grown to ensure uniform hardness and low friction. The technology for synthetic corundum dates back to the late 1800s.

Uses & applications

Used as 'jewel bearings' in mechanical watch movements to minimize friction at high-wear pivot points. Also used in high-precision instruments and lasers.

Geological facts

The '21 Jewels' text on this Seiko dial indicates the number of synthetic rubies used in the movement to reduce wear. Using jewels significantly extends the lifespan and accuracy of the automatic mechanism.

Field identification & locations

Identified in field/horology by the 'Jewels' marking on the dial or movement. They appear as small, pink/red translucent discs or cups when the watch back is opened. Found in most high-quality mechanical watches worldwide.

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