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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.

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.