Corundum Value, Price, and Jewelry Information

Identifying Characteristics

• 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.

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.

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.