About London Blue Topaz :-London blue topaz is a medium to dark grayish blue, sometimes described as "steely" or "inky". Many London blue gems have a slightly greenish tone when viewed from certain angles. The reason why 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 on the market is produced by treating white topaz with radiation. 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.