Blue topaz begins "life" as colorless or very lightly tinted natural topaz crystals which are then irradiated to change the color to blue and heated to stabilize the change. Neutron bombardment in a nuclear reactor produces the deep slightly greenish or greyish London Blue, while electron bombardment in a linear accelerator results in the light aqua-like blue known as sky blue. Combinations of both treatments produce the highly saturated Swiss and electric blues. If neutron bombardment has been used, there is residual radioactivity, and the gems must be held, up to a year, before they have "cooled" enough to be worn.
The modest value of most blue topaz creates little incentive in the market for sythetic blue topaz, although it has long been simulated by synthetic spinel. More lucrative and popular are the various vapor deposition or diffusion coatings that create "mystic topaz" and teal, red and sea green colors. Such stones are attractive but the treatment is not permanent, with their extremely thin coating they must be handled very gently as any scratch or abrasion can mar the surface layer.
Whatever the color, topaz has some wonderful gem qualities due to its high refractive index and its ability to take a fabulous polish. The fact that the rough is available at moderate prices in rather large, clean pieces means that many cutters choose this gem for their fancy or non-traditional cuts. At hardness 8 topaz makes a good gem for occasional wear rings, pendants, earrings or brooches, but, alas, the ready cleavage of this gem makes its use in daily wear rings very risky.
In general, blue topaz is modestly priced, although, due to recent shortages, the London blue color has outstripped the others in value. The shortage is due to poor economics: reactor time is expensive and there are more profitable gems which can be treated without the need for such an extensive holding period. There is no special premium for larger stones in this variety and clarity is routinely expected, so included pieces should be extremely inexpensive. Cut often adds as much or more value to the piece than the material itself. Spectacular cuts and carvings are available at generally reasonable prices.