The corundum family of gemstones consists of ruby and sapphire. Corundum is very compact, dense, and lacks gemstone cleavage. It's also the second hardest natural mineral after diamond. Gem-quality corundum is also quite rare. These factors make both varieties of corundum some of the most highly desired jewelry stones. Ruby is the red variety of corundum. All other colors of corundum, including colorless, are called sapphires.
Rubies and sapphires are some of the most valuable and popular of all gemstones. Despite the enormous size of these stones in museums and royal collections, most corundum gems available for sale are usually of more modest size. A 3-4 carat ruby of fine quality would be a rare and very expensive gem in today's market. Sapphires over 5 carats, clean and with good blue color, are similarly rare and also valuable. Relatively speaking, there is an abundance of good quality small sapphires but not rubies. Very few known ruby deposits can be actively worked, while there are many more sapphire deposits being actively mined.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Is a Variety of||Corundum|
|Varieties||Color Change Sapphire, Padparadscha, Star Sapphire|
|Crystallography||Hexagonal (trigonal). Crystals common, often barrel-shaped, prisms with flat ends, sometimes bipyramidal; also massive, granular, in rolled pebbles.|
|Colors||Pinkish red, medium to dark red varieties are called ruby. All other colors are called sapphire, including colorless, white, gray, blue, blue-green, green, violet, purple, orange, yellow, yellow-green, brown, golden amber, peachy pink, pink, and black|
|Luster||Vitreous to adamantine|
|Polish Luster||Vitreous to subadamantine|
|Density||3.99-4.1; usually near 4.0|
|Cleavage||None. Fracture conchoidal; frequent parting. Slightly brittle, usually tough.|
|Stone Sizes||See “Stone Sizes” below.|
|Luminescence||Intense and distinctive in identification. See entries for ruby and sapphire.|
|Spectral||The spectrum of corundum can be used diagnostically. See entries for ruby and sapphire.|
|Transparency||Transparent to Opaque|
|Phenomena||Asterism, color change, chatoyancy|
|Formula||Al2O3 + Fe, Ti, Cr.|
|Pleochroism||Very pronounced in colored corundum gemstones. See entries for ruby and sapphire.|
|Optics||RI: o = 1.757-1.770; e = 1.765-1.779 (usually 1.760, 1.768); Uniaxial (-)|
|Etymology||From the Sanskrit kurivinda for ruby.|
|Occurrence||Metamorphosed crystalline limestones and dolomites, as well as other metamorphic rock types such as gneiss and schist. Also, igneous rocks such as granite and nepheline syenite. Gem-quality corundum is often found in placer deposits. Non-gem corundum is abundant throughout the world.|
|Inclusions||In general, Burmese, Thai, and Australian blue sapphires contain crystals of plagioclase feldspars, orthoclase, niobite, columbite, calcite, monazite, zircon, apatite, fergusonite, and thorite. Tanzanian sapphires contain crystals of chlorapatite, pyrite, magnetite, biotite, graphite, phlogopite, zircon, and spinel. See “Identifying Characteristics” below.|
- Natural: Silk, rutile needles, usually crossing at a 60 degree angle. Zircon crystals with halo of dark fractures, fingerprints, hexagonal growth lines, color zoning.
- Synthetics: See Identifying Inclusions and Synthetic Gemstones and Their Identification.
|Characteristics of Corundum from Various Localities|
|Jauru, Matto Grosso||Sapphire||dark blue||1.762||1.770||0.008||3.95-4.05|
|Japan||Ruby||purplish-red to pink||1.761||1.769||0.008||3.89|
|Umba River Valely||Sapphire||red-brown||1.763-1.765||1.771-1.773||0.008||3.99-4.06|
|Note: Colorless: e=1.759 -1.761; o=1.768 -1.769;blue and green: e = 1.762 -1.770; o= 1.770-1.779. Among sapphires the green gems are more iron-rich, have higher indices.|
Star corundum gemstones show asterism, or a “star effect.” This phenomenon is created by the inclusion of rutile needles within the host corundum crystal. The rutile needles orient themselves according to the hexagonal symmetry of the corundum, and reflections from these needles provide a chatoyancy. When such material is cut into a cabochon the sheen is concentrated along the top of the stone into three white lines crossing at 120° angles, creating a six-rayed star. Very rarely there are two distinct sets of needles oriented according to the first and second order prisms of the corundum (30° apart), resulting in a strong, 12-rayed star.
For more information on corundum inclusions, see Identifying Origins of Rubies and Sapphires.
- Heat treatment: improves clarity and sometimes color, also used to improve asterism in ruby. Common, often undetectable. Detect with magnification, (halos around included crystals.) Also, if inert and no absorption lines, stone is probably heat treated.
- Diffusion: improves color or asterism, occasional to common for blue sapphire and star ruby, rare for other colors, stable, detect with immersion and magnification.
- Thin film coating; enhances or changes color, rare, easily scratched, detect with magnification.
- Irradiation: produces yellow and orange from light colored material. Occasional, stable, undetectable.
- Lattice diffusion: produces padparadscha color from light material. Occasional, stable, detect with immersion and magnification.
- Oil, dye, glass, or other fillers: Hides fractures and improves color. Glass increases weight. Occasional, most stable but oil can dry out. Detect with immersion and magnification.
- Fracture healing: done by placing natural rubies in flux growth medium. Fills fractures, improves clarity. Stable, common. Detect flux with magnification.
- Lead glass filling: Fills fractures, improves clarity. Stable, common. Detect flux with immersion.
See corundum treatments for more information.
Rubies and sapphires are found worldwide. For location details, see Identifying Origins of Rubies and Sapphires.
Gem-quality corundum is occasionally found in the Czech Republic, Finland, Greenland, Nepal, Norway, Pakistan, Russia, and Slovakia.
Sapphires, in general, reach a far greater size than rubies. A ruby of 30 carats is a great rarity, whereas sapphires in museum collections weighing hundreds of carats are not uncommon. The largest rubies come from the chrome-zoisite matrix in Tanzania, but these are not really of gem quality. Fine gem rubies of large size occur in the Sri Lankan gravels, with smaller ones from Burma and Thailand. Enormous sapphires of fine color and transparency have been found in the gem gravels of Sri Lanka and Burma. (Most are from Sri Lanka). A 1400 gram ruby was found in Yugoslavia (Prilip) but was not gemmy. Malawi material (sapphire) reaches a size of about 12 carats. Large sapphires have been found in Australia. Montana sapphires over 1 carat are very rare, but the blue ones in this size range are magnificent.
Specific gemstones of significant size are described in the ruby and sapphire entries.
Consult our gemstone care guide for recommended cleaning methods.