Species and varieties
Corundum is the mineral species comprising the gem varieties ruby (red) and sapphire (all other colors). Composition is aluminum oxide (Al₂O₃); Mohs hardness is 9, second only to diamond among naturally occurring minerals. Specific gravity ranges from 3.97 to 4.05.
Color and chemistry
Color in corundum is produced by trace-element substitution. Chromium produces ruby red; iron and titanium together produce the classic blue sapphire color. Iron alone produces yellow sapphire; vanadium, color-change material; chromium with iron, padparadscha-orange.
Sources
Commercial rough is produced in:
- Sri Lanka — long-established source; the Ratnapura and Elahera regions produce a wide range of colors.
- Madagascar — Ilakaka deposits, since the 1990s, produce significant blue and pink material.
- Mozambique — Montepuez region, since approximately 2009, produces commercially significant ruby.
- Myanmar — Mogok valley, historic source of "pigeon's blood" ruby.
- Australia — central Queensland, predominantly blue sapphire of darker tone.
- Montana, USA — Rock Creek and Yogo Gulch, distinctive blue and parti-colored material.
- Kashmir — historic source of velvety-blue sapphire; very limited current production.
Treatments
Treatment conventions for corundum are extensive:
- Heat treatment is standard and rarely separately disclosed for sapphire.
- Beryllium-diffusion alters color via lattice diffusion at high temperature; it is detectable in the laboratory and should be disclosed.
- Lead-glass filling of fractures is common in lower-grade ruby; it should be disclosed.
- Synthetic corundum (Verneuil, Czochralski, flux) has been commercially available since the early 20th century and is identifiable in the laboratory.
Disclosure requirements vary by jurisdiction and seller.
Lapidary considerations
Corundum is hard but brittle relative to its hardness; it cuts cleanly with diamond grits but requires patience at the polish stage. Manufacturer cutting guidance for corundum-class material typically calls for a 360-mesh cutting lap, a 3,000-mesh pre-polish, and a 50,000-mesh or finer polish.
Heat-treated material can show color zoning and silk inclusions visible at 10× magnification. The species is dichroic, and orientation during preform layout affects the apparent color of the finished stone.