Red variety of corundum. Nowadays, sellers often like to call even pink, violet, and orange-red corundum rubies instead of sapphires. From a cutting point of view, the nomenclature is not of great importance.

Physical properties:

Chemistry: Al2O3
Hardness: 9
SG: 4.00
Cleavage: None
Fracture: Sub-conchoidal to uneven, tough
Thermal sensitivity: None

Optical properties:

RI: 1.76-1.77
Dispersion: 0.018 (medium)
Double refraction: 0.008 (weak)
Pleochroism: Strong, purplish-red and orangish-red
Critical angle: 34.5°

Facet angles (pavilion/crown):

Vargas: 42°/40°
Olson: 39°/37°
Soukup: 42°/37°
Roth: 42°/37°
MDR: 42°/37°
Schlagel: 40°/36°
Hashnu: 42°/40°
Sinkankas: 40°/40°
GIA: 42°/37°
Perkins: 42°/37°
Raytech: 40°/38°
Cornwall: 40°/34°
Weikoff: 42°/38°
Carroll: 40.8°/33.4°

Cutting lap:

All grain sizes below 1200 (just 1200 often causes problems with "glazing"). Pre-polish with 3k-14k.

Facet Polishing:

Vargas: 100k diamond on ceramic disc or tin
Olson 14k diamond on tin or copper
Soukup: 14k diamond on copper, zinc, or tin
Christiansen: 50-100k diamond on tin or copper
Herbst: Diamond on metal or ceramic
Perkins: Diamond on BATT
Raytech: 14k or finer diamond on Fast Lap, ceramic, tin, zinc, copper, or phenolic
MDR: 14-50k diamond on tin

Cabochon Polishing:

Olson: 8-14k diamond on wood
Christiansen: 50-100k diamond on wood
Cox: 8-14k diamond on wood, plastic or crystalpad


For best color and yield.


Can be treated with high heat and with lead glass that seeps into cracks and voids, which improves color and transparency, but the stone can become brittle and have streaks/patches of glass on facet surfaces that cannot achieve the same high gloss as corundum. Beware of synthetic ruby that has been made to look like natural rough stones.


Corundum is very hard and generates high heat when polished. Have a small cup of water on hand and dip the stone every 5 seconds. Corundum can also be polished with aluminum oxide, which some cutters finish with to give the polish an extra boost.

Ruby rough

Ruby on Gemdat

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