Topaz is becoming the popular gemstone with the widest variety of human influence on its appearance. When shopping for topaz jewelry, you rarely come into contact with a gemstone that resembles something from the earth. Almost every topaz you can find has been treated in a way: you can get a clue about those treatments by observing the stones' color.
Pale Blue: If you see a pale blue stone and it is very expensive, there is a small chance it could be natural. Otherwise the color of these stones is usually produced through heat treatment.
Intense Blue: "Electric blue", "Swiss Blue", "London Blue" and any other very bright blue color you see is usually the result of two treatments: irradiation and then heat. In the case of irradiation, the gemstones need to "cool off" for up to 2 years before they can be worn due to their radioactivity after the treatment. It is so difficult to govern or account for these procedures, that Europe is beginning to take steps to ban gemstones treated in this way. These gemstones should be purchased with awareness and caution.
Brown: Brown topaz is the second most common color found in natural, untreated topaz. Brown topaz can also be the result of treatments, although it is not common since the cost of the treatment couldn't be made up in the price of the stone. So a brown topaz is likely to be natural and unheated.
Yellow: Yellow topaz also occurs in nature, however bright colors are usually the result of treatment. Extra caution is needed when looking at a piece of "yellow topaz" however, because yellow quartz (citrine) is often incorrectly given that label. "Yellow Topaz Quartz" is less deceptive, but it is not topaz. Since citrine is less valuable than topaz, it is important to make the distinction. You can tell with a scratch test. Find a spot on the girdle (thin edge between the top and bottom facets), and scratch the stone with something of at least 7 on the Moh's Scale (e.g. garnet). If the stone scratches, you know it's not topaz.
Pink: Pink is the most valuable color of topaz. However, in nature pink shades of topaz are extremely rare. Traditionally, pink topaz was created by heating yellow topaz. This has been done for hundreds of years and is a fairly stable treatment. Recently, though, very "hot" or intense, dark colors of pink topaz are being achieved through "coating". A coat of colored film is put on the gemstones surface and then it is exposed to high pressure or heat. The color absorbs into the stone but only in a shallow layer. This means that if the stone is severely scratched or re-polished or chipped, the natural (usually white) topaz will show.
Iridescence: More and more colors and varieties of topaz are available daily with an iridescent, incandescent look to them. The most common and popular of these is "Mystic Topaz". There is also "Rainbow topaz", "Mystic Firelight", "Azotic", etc. These stones are created when colorless topaz is coated with titanium oxide (a film like the one on a camera lens) and then exposed to pressure. The treatment is referred to as "coating". Since the color only seeps through to a thin surface layer of the gemstone, it tends to wear off over time.
Colorless: This is the one form of topaz that you can count on as not being treated. Colorless or "white" topaz is the most abundantly found in nature. It is therefore, not very expensive. Many of the colored topaz stones you can buy started out as colorless before their treatment.
Research topaz treatments and ask your jeweler questions. Be informed so you know what you're buying.
Source: http://www.therealarticles.com/Article/Shopping-for-Topaz-Jewelry--Treatments--Fakes--and-Radioactivity/153987
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