London Blue Topaz
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| London Blue Topaz |
Blue gemstones with good hardness and brilliance tend to be rare and fairly expensive. This is especially true of the darker blue gems, such as sapphire, tanzanite and spinel. There are other, less expensive, gems in a saturated deep blue, including kyanite, lapis lazuli, iolite and fluorite. But these are much softer stones that are not suitable for everyday jewelry such as rings.
It is not surprising then that 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.
London Blue Topaz is a medium to dark grayish blue, sometimes described as "steely" or "inky." Many London Blues have a slightly greenish tone when viewed from certain angles.
The reason that blue topaz is so reasonably priced is that topaz is a very abundant material. But natural topaz occurs mainly in white (colorless) and brown; natural blue topaz is actually very rare. Virtually all the blue topaz in the market is produced by treating white topaz with radiation.
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| Natural White Topaz |
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.
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