Gemstones are frequently subjected to treatments to alter or enhance their color. Heat, radiation, dyeing and oiling are four of the more widely used procedures for altering a gem’s color. While this is a common practice, buyers should be advised when a stone has been treated.
When heat is applied to some gemstones it can lighten, darken or completely change their color. The change is usually permanent. Jewelers have been using this process for hundreds of years, and most rubies and sapphires undergo heat treatments. A stone that has an excellent color and has not been subjected to heat treatment can command a higher price than one that has.
• Amethyst – heat applied to amethyst can result in yellow or green stones
• Aquamarine – heat can darken the shade of blue
• Sapphire – heat lightens the blue, can turn violet stones to pink, or deepen a pale color
• Tanzanite – heat is used to produce brilliant blue tanzanite from a dull brown stone
• Topaz – heat produces pink or blue stones
• Tourmaline – heat lightens the dark shades of tourmaline
Like heat, radiation treatments can alter gemstone color. Color changes from radiation are not always permanent – it depends on the stone.
Radiation is sometimes used on these gemstones:
• Aquamarine – together with heat, radiation can darken the shade
• Diamond – radiation can take the color from off-white to another color, like green
• Sapphire – radiation can create gold colored sapphires
• Topaz – radiation takes the stone from clear to blue
Oiling is used mainly on emeralds. Emeralds are prone to fine cracks which can appear as unattractive white lines. The oil fills these cracks so that the white is no longer apparent. The treatment usually lasts for years, however hot ultrasonic cleaners or organic solvents like gasoline or those in paint remover can remove the oil from the emerald. In such a case, the stone can be oiled once again.
This is more of a caveat emptor-type thing. In some cases, where the gem is set into the gold, and the back is closed so that you can’t see the underside of the gem, colored foil is used beneath the gem to give it a richer color.