gemstone

Ruby (Corundum)

Aluminum Oxide (Al₂O₃:Cr)

Hardness: 9 on Mohs scale; Color: Deep pinkish-red to purplish-red; Luster: Vitreous to adamantine; Crystal system: Trigonal/Hexagonal; Cleavage: None (shows parting); Specific Gravity: 3.97–4.05.

Hardness: 9 on Mohs scale
Color: Deep pinkish-red to purplish-red
Luster: Vitreous to adamantine

Identified July 23, 2026More gemstone →

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Physical properties

Hardness: 9 on Mohs scale; Color: Deep pinkish-red to purplish-red; Luster: Vitreous to adamantine; Crystal system: Trigonal/Hexagonal; Cleavage: None (shows parting); Specific Gravity: 3.97–4.05.

Formation & geological history

Formed in regional metamorphic rocks such as marble and amphibolite, or in igneous rocks like syenite, where aluminum-rich fluids under high pressure and temperature react in silica-deficient environments.

Uses & applications

Cut and polished for high-end gemstone jewelry; historically used as pivot bearings in mechanical watches and in early laser technologies.

Geological facts

Ruby is the red variety of the mineral corundum. Pure corundum is colorless; the intense red hue is caused by trace amounts of chromium replacing aluminum atoms in the crystal lattice.

Field identification & locations

Look for extreme hardness (scratches quartz and topaz), high density, hexagonal growth lines, and strong UV fluorescence. Found in regional metamorphic belts globally, including East Africa.