Blue topaz has become one of the top selling gems in the jewelry business, since blue topaz is unusual in offering excellent hardness (8 on the Mohs scale) and brilliance at a very reasonable cost.
Blue topaz can be found in both lighter and darker tones, usually known in the trade as Sky Blue Topaz, Swiss Blue Topaz and London Blue Topaz. As in the case of other blue gems, the more saturated blues tend to have a higher value. So in topaz it is the London Blue that usually regarded as the most valuable. There are two important things to know about blue topaz. The first thing is that while topaz is very hard, it is not the most durable gemstone. That's because it has perfect cleavage, a property it shares with diamond. That means it can be chipped or split by a sharp blow, so it should be protected from hard knocks.
The second important thing is that topaz does not occur naturally in the deeply saturated blues you find in the market today. Blue topaz in nature is very rare indeed, and tends to a very pale blue. The vivid blues available in the market have all been produced by treating white topaz -- first with irradiation, then with heat. The color change is permanent and stable, but recently there has been some controversy about the safety of this treatment for the consumer.
The reason that the color of topaz can be changed by irradiation is a function of the special way that topaz gets its color. Most gems, such as sapphire, are colored by trace elements such as iron or titanium. Some gems, such as peridot, are colored by elements in their essential chemical composition. But topaz is unique in that the color results from so-called color centers, which are imperfections in the crystal lattice that change the way the crystal absorbs light.
London Blue Topaz is typically produced by exposure to radiation in a nuclear reactor. When topaz is exposed to fast neutrons, the radiation changes the color centers, producing the deep blue color. Subsequent heat treatment is often used to lighten the inky color. Material treated this way is likely to be radioactive and may require several months of storage before the radioactivity decays to safe levels. There are very strict rules in place to protect not only consumers but also the cutters and gem dealers who handle these gems on a daily basis."
The valuable information about Topaz gemology in this article is provided by Gem Select .