Where topaz is found
Chemistry: Al2SiO4 (F, OH)2 + Cr
Refractive index: 1.607 - 1.627
Specific gravity: 3.53 - 3.56
Cleavage: Perfect one direction
Heat sensitive: No
Wearability: Very Good
Special care instructions: None
Enhancements: Pink or Red may be heat treated. Most blue topaz has been irradiated and heat treated.
The term "precious" topaz was originally used to distinguish yellow and orangish topaz from other gems such as some citrines and smoky quartzes, which had erroneously been referred to in the past as "Maderia topaz' and "smoky topaz."
The confusion derives from the Brazilian word "topazio," which means yellow and was used generically by miners.
Most precious topaz still comes from Brazil.
Topaz of any type is an excellent jewelry stone, and it is historically one of the most valuable gemstones.
With its relatively high refractive index, hardness of 8 and with no particular sensitivity to chemicals, it can be used, with appropriate care, in any jewelry application.
Although perfect cleavage does present a caution, this is mostly solved in the cutting stage --cutters generally orient the table of the stone 5 -10% off the cleavage plane, which results in a pretty stable stone during cutting and wearing.
It is necessary to protect the gem from hard knocks and to avoid steamers and ultrasonic baths though.
The subject of enhancement in the topaz family is a complicated one.
Still, for the most part, except for colorless stones, it is prudent to assume that some form of heat and/or irradiation has been used on stones before cutting.
The color of precious topaz is generally heat and light stable, unlike some natural and enhanced types of brown topaz, which can fade dramatically in strong light.
The hue and saturation of color are the primary determiners of value in this variety.
In general, the more pink or red mixed in with the yellow or orange, the higher the value.
Most precious topaz is native cut in Brazil, so custom cuts are potent value enhancers.
Size comes at a premium in all the topazes except blue and colorless.
There's an exponential jump in value in stones larger than 5 carats and again for stones larger than 10 carats.
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 grayish London Blue.
Electron bombardment in linear accelerator results in the light aqua-like blue known as sky blue.
Combinations of both treatments produce 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 synthetic blue topaz, although a synthetic spinel has long simulated it.
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 is rather large, and clean pieces mean 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.
In general, blue topaz is modestly priced, although, due to recent shortages, the London blue color has outstripped the others in value.
The deficit is due to poor economics: reactor time is expensive, and there are more profitable gems that can be treated without the need for such an extensive holding period.
There is no exclusive premium for larger stones.
In this variety, 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.