In using the term "precious topaz", I'm referring to stones of a rich yellow to a medium peachy orange color. In the future I'll cover Imperial Topaz which, to my way of thinking, is in a separate category of only those stones with very saturated reddish orange color. There will also be a later section on other topazes, such as pinks, browns and colorless stones.
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 a good jewelry stone and it is historically one of the most important gemstones. With its relatively high refractive index and hardness of 8, with no special 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. All that is necessary is to protect the gem from hard knocks and to avoid steamers and ultrasonics in cleaning.
The subject of enhancement in the topaz family is a complicated one, but 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 prior to 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. In my opinion, when you have a stone that has that good precious topaz color, there is no chance that it will be mistaken for a citrine, a yellow sapphire, or any other gem the color is so distinctive!
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, 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 synthetic 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.
Lately on eBay there has been a glut of loose gemstones available, most coming from Asian dealers, which are labeled "Imperial topaz". These stones tend to be some variation of brown, gold, yellow, orange, or "peach" or "salmon" colors as the dealers call them. The photographs of these stones are manipulated to a large degree. The actual stones seldom look anything like the listing images. Sometimes, prices in the hundreds of dollars are paid for these stones.
The sad truth is that very few, if any of these stones are true Imperial topaz. Most of them are what the Brazilian topaz miners call, in the Portuguese language, "topazio marrom", brown topaz. Miners sell it for pennies per carat, and it makes its way to the gem cutting centers of Thailand where it is cut into various sizes and shapes for pennies a carat. The true Imperial topaz is always cut in Brazil, as the Brazilians will not sell the "rough" topaz to other countries. Some of the Thai cutting is quite good, but the material being cut is nearly worthless. To make matters worse, most if not all of it, after exposure to sunlight, will fade to colorless topaz. I have tried this myself, and I now have a nice collection of "white" or colorless topaz.
Be careful of intensely pink stones which are sold as Imperial topaz. Most of these stones are in fact coated colorless topaz. The "Azotic" coating used to create the pink color utilizes heat and titanium in a process called "vapor deposition" to produce the pink color, usually a shocking pink.
True Imperial topaz came from one of two places: either mines in the Ural Mountains of Russia, or the Ouro Preto district of Brazil. Such stones come in shades of "sherry gold", usually with some red near the tips, or various shades of pink, red, or even purple. The Russian mines have long been depleted and the Brazilian mines have been closed for years. The true Imperial topaz is a rare stone, and seldom comes up for sale.
One final word: do not confuse "Imperial topaz" with "precious topaz", which has yellowish to gold tones. Precious topaz is much more common than Imperial topaz, and consequently worth much less.
The hue and saturation of color is the primary determiner 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 strong 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.
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 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.