Blue topaz has become one of the most popular gemstones on the market today, due to its clarity, durability, availability and affordable cost. Yet it is a shade of topaz rarely found in nature. The stone's watery blue color is most often created through a combination of heat treatment and irradiation.

Topaz is one of the well-known pegmatite minerals that also includes beryl and tourmaline. Blue topaz is the birthstone for December - and is also the recommended gemstone for couples celebrating their fourth wedding anniversary.

Blue topaz has a definite, uniform color ranging from sky blue to Swiss blue. It is sometimes

confused with the more costly aquamarine - yet whereas aquamarine sometimes has a greenish-blue or bluish-green tint, blue topaz will always look blue or bluish gray. The one exception is for surface-enhanced topaz, a process introduced in 1998 that enhances the stone's appearance and brings out colors such as blue to greenish-blue to emerald green.

Most blue topaz starts life as a colorless or slightly tinted topaz from places like Brazil, Mexico, Nigeria, Sri Lanka and China. It is then irradiated (to incite the color change) and heated (to stabilize the change). The result is a permanent aqua shade. To get deep blue color out of topaz, treaters use neutron bombardment in a nuclear reactor and market the final product under the name "London blue." In fact, neutron bombardment is the only means by which to produce smaller calibrated stones with deep color.

Despite the stone's exposure to irradiation, experts say it poses no health hazard whatsoever to the wearer.

In addition to blue, the stone comes in a variety of colors, including golden yellow, orange-yellow, reddish-orange, sherry red, deep pink, honey brown, light green, and many shades in between.