Very few specimens of corundum have a natural color within the range required for a ruby. Very few also have the clarity required to produce a nice faceted stone. Long ago, people who prepared gem materials for cutting began experimenting with ways to improve their color and clarity.
Physical Properties of Corundum:
| Chemical Classification | Oxide |
| Color | A gem corundum with a dominant red color is a ruby. Any other color of corundum is a sapphire. |
| Streak | Colorless (harder than the streak plate) |
| Luster | Adamantine to vitreous |
| Diaphaneity | Transparent to translucent |
| Cleavage | None. Corundum does display parting perpendicular to the c-axis. |
| Mohs Hardness | 9 |
| Specific Gravity | 3.9 to 4.1 (very high for a nonmetallic mineral) |
| Diagnostic Properties | Hardness, high specific gravity, six-sided crystals sometimes tapering to a pyramid, parting, luster, conchoidal fracture |
| Chemical Composition | Al2O3 |
| Crystal System | Trigonal |
| Uses | Historically used as an abrasive. Specimens with pleasing colors have a long history of gemstone use. |
Heating:
Heating corundum crystals under controlled conditions can improve or intensify their color. Heating can also remove inclusions by causing them to dissolve, making them less visible and improving the clarity of a gem.
Most rubies in the market today have been heated to improve their color and clarity. This heat treatment is normal and expected in the gem trade, but a seller should disclose the treatment to a buyer in advance of a sale.
Fracture Filling:
One of the early treatments was to fill surface-reaching fractures with oils, waxes, or resins. These treatments filled pits and fractures on the surface of the gem and improved their appearance. However, these treatments are not permanent because the oils can be washed out, and the waxes and resins can crack and fall out with age – even with special care. They produce a temporary improvement in the appearance of the stone and are mainly done to produce a quick and profitable sale.
A more permanent type of fracture treatment is to fill the fractures with minor amounts of flux, glass, or another durable material. These enter the fractures during the heat treatment process. When the stone cools, a permanent filling of the fracture was accomplished. These treatments reduce the visibility of the fractures and improve the clarity of the gems. They also might improve the durability of some stones. This type of treatment is generally acceptable but should be disclosed to the buyer.
A much more aggressive treatment is to heat the gem to a very high temperature and inject glass or flux into the fractures. The temperature of this treatment can be high enough that some of the ruby melts and mixes with the fracture-filling materials. The result of this treatment is an altered stone with an improved appearance. But the stone now contains an unknown and possibly significant amount of non-ruby material. If these gems are sold “by the carat,” the buyer might pay a significant portion of the price for non-ruby material. Many people believe that these treatments produce man-made composite materials that should not be called “ruby.”