The treatment and enhancement of gemstones has existed for thousands of years and the first documentation was presented by Pliny the Elder. Although many of these enhancements are virtually undetectable, the process provides us with countless beautiful gemstones which would possibly otherwise be virtually unknown.
Some heat treatments will bring about a very subtle change of colour whilst others can be quite dramatic. One familiar gemstone which is heat treated is Topaz which after being heat treated ends up being a shade of blue and is subsequently marketed as Blue Topaz.
Although colour can be altered by time, Amethyst, Rose Quartz and Kunzite can become paler when exposed to direct sunlight, natural colour changes are actually quite rare. Amethyst is well known for being heat treated which changes its colour from a distinctive purple to light yellow, reddish brown, green or a milky white. In fact most Citrine and Prasiolite available on the market are heated Amethyst as both of these gemstones in their natural form are actually pretty rare. Prasiolite which is also sometimes called Green Amethyst, is a variety of green Quartz which gets its colour from impurities of iron.
Colours which are not considered to be particularly appealing can be changed to make a gemstone more desirable. Greenish Aquamarine can be changed to light blue, Tourmaline which is too dark can be lightened and even Rubies and Sapphires are heat treated to improve their colour. Colours can also be improved by radium and X-ray treatment and examples of this are London Blue and Swiss Blue Topaz. In the case of porous gemstones such as Lapis Lazuli, Turquoise, Pearls and Agate, colours are improved by the addition of pigments, oils, resins and wax and these treatments have been practiced for thousands of years.
Further reading:
Gemology Online
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