Corundum Value, Price, and Jewelry Information

CORUNDUM: Sapphire, Montana (1.35, 1.77, 1.47, 1.66 // 1.19, 1.10, 1.40, 1.22, 1.30 // 1.03, 1.10, 0.96, 0.95, 2.30). Photo © Joel E. Arem, PhD, FGA. Used with permission.

Corundum

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.

Corundum Value

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.

Corundum Information

Data Value
Name Corundum
Varieties Ruby, Flux-Grown Ruby, Geneva Ruby, Star Ruby, Verneuil Ruby, Sapphire, Color Change Sapphire, Padparadscha Sapphire, Star Sapphire
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
Crystallography Hexagonal (trigonal). Crystals common, often barrel-shaped, prisms with flat ends, sometimes bipyramidal; also massive, granular, in rolled pebbles.
Refractive Index 1.757-1.779
Luster Vitreous to adamantine.
Polish Luster Vitreous to subadamantine.
Fracture Luster Vitreous.
Hardness 9
Wearability Excellent
Fracture Conchoidal; frequent parting
Specific Gravity 3.99-4.1; usually near 4.0
Birefringence 0.008-0.009
Cleavage None
Dispersion 0.018
Stone Sizes See “Stone Sizes” below.
Heat Sensitivity no
Luminescence Intense and distinctive in identification. See gem listings for Ruby and Sapphire.
Luminescence Present Yes
Luminescence Type Fluorescent, UV-Long, UV-Short, X-ray Colors
Enhancements Heat treatment; diffusion; thin film coating; irradiation; lattice diffusion; oil, dye, glass, or other fillers; fracture healing; lead glass filling
Typical Treatments Dyeing, Flux Healing, Fracture/Cavity Filling, Heat Treatment, Irradiation, Lattice Diffusion, Oiling, Surface Coating
Transparency Transparent to Opaque
Absorption Spectrum The spectrum of corundum can be used diagnostically. See gem listings for Ruby and Sapphire.
Phenomena Asterism, color change, chatoyancy.
Formula

Al2O3 + Fe, Ti, Cr.

Pleochroism

Very pronounced in colored corundum gemstones. See gem listings for Ruby and Sapphire.

 

Optics

RI: o = 1.757-1.770; e = 1.765-1.779 (usually 1.760, 1.768); Uniaxial (-)

Optic Sign 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.

CORUNDUM: Sapphire, Sri Lanka (184, in gold pendant with diamonds)

Sapphire and ruby are prized jewelry stones. CORUNDUM: Sapphire, Sri Lanka
(184, in gold pendant with diamonds). Photo © Joel E. Arem, PhD, FGA. Used with permission.

Identifying Characteristics

Characteristics of Corundum from Various Localities
Locality Variety Color e o Birefringence S.G. Comments
Australia Sapphire Light blue 1.761 1.769 0.008 4.02
dark blue 1.763 1.772 0.009 3.99
green 1.763 1.772 0.009 4.00
yellow 1.765 1,774 0.009 3.97
yellow-green 1.767 1,775 0.008 3.99
golden yellow 1.763 1.772 0.008 4.01
Brazil
Jauru, Matto Grosso Sapphire dark blue 1.762 1.770 0.008 3.95-4.05
Burma Ruby Finest Red 1.760-1.769 1.768-1.778 0.008-0.009 3.996
Sapphire dark blue 1.762 1.770 0.008 4.00
India
Kashmir Sapphire fine blue 1.762 1.770 0.008 3.99
Colombia Sapphire blue, violet 1.762 1.770 0.008 3.99-4.02
Japan Ruby purplish-red to pink 1.761 1.769 0.008 3.89
Malawi Sapphire various 1.760 -1.761 1.770 0.009
Ruby red 1.762-1.763 1.771-1.772 0.009
Nepal Ruby red 1.760 1.768 0.008 3.98
(Taplejung district)
Pakistan Ruby red 1.762 1.770 0.008 3.99
Hunza Valley
Sri Lanka Ruby red 1.761-1.763 1.769-1.772 0.008 3.99-4.00
Sapphire blue 1.757 -1.760 1.765-1.768 0.008 4.00
yellow 1.760-1.761 1.768 -1769 0.008 3.99-4.01
green 1.765 -1.770 1.773-1.779 0.008-0.009 4.00-4.01
Tanzania Ruby red 1.764 1.772 0.008 3.99
Longido Sapphire orange 1.760-1.763 1.768-1.772 0.008-0.009 3.99
Umba River Valely Sapphire red-brown 1.763-1.765 1.771-1.773 0.008 3.99-4.06
Thailand Ruby red 1.760-1.764 1.768-1.772 0.008 4.01
Ruby dark red 1.768 1.776 0.008 4.00
Yugoslavia Ruby red ~1.765 3.80-3.98
Prilip
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.
CORUNDUM: Star ruby, Sri Lanka (2.75), Burma (2.6, 0.62), India (8.4

CORUNDUM: Star ruby, Sri Lanka (2.75), Burma (2.6, 0.62), India (8.4). Photo © Joel E. Arem, PhD, FGA. Used with permission.

Inclusions

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.

CORUNDUM: Sapphire-Star sapphire, Sri Lanka (31.87)

CORUNDUM: Sapphire-Star sapphire, Sri Lanka (31.87). Photo © Joel E. Arem, PhD, FGA. Used with permission.

Enhancements

  • 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.

Sources

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.

Stone Sizes

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.

Care

Consult our gemstone care guide for recommended cleaning methods.