Ruby (corundum, red variety)
gem · uncommon
Quick facts
- Formula
- Al2O3
- Hardness
- 9
- Luster
- Vitreous to subadamantine
- Colors
- Red to purplish red (colored by Cr), darker brownish-red in some stones
Ruby is the red variety of corundum, colored primarily by chromium. It is one of the most durable and valued colored gemstones, sourced from Myanmar, Mozambique, Sri Lanka, Tanzania, and others; many stones are treated to improve color/clarity.
Exceptionally hard (Mohs 9), strong red pleochroism, possible red fluorescence, and high heft for size. Under magnification, natural rubies may show rutile silk or growth zoning; curved striae indicate flame-fusion synthetic.
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