Vivid, clear gemstone colors are scarce in nature and therefore expensive. Treating stones with heat and radiation is common to increase the availability of affordable stones in desirable colors. Colored gemstones sold as "natural" should have a certificate of authentication verifying this to support their higher price.
Heat
People have heat treated gemstones for hundreds of years. Heating amethyst yields yellow citrines. It is routine to heat sapphires, rubies, aquamarines and tanzanite to deepen color, remove undertones or improve the clarity of the stones.
Radiation
Radiation is a common way to change or enhance gemstone colors. Fancy pink, yellow and other colored diamonds are more widely available due to radiation treatment. Nearly all blue topaz is originally white topaz that has been irradiated. Treatments frequently combine radiation and heat to maximize appearance.
Oiling and Filling
Oiling is a frequent treatment for emeralds using natural oils or synthetic polymers to improve perceived depth of color. Stones like turquoise and opal are frequently filled in to smooth out naturally occurring cracks in the stones and make them more durable.
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