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.
|Varieties||Mystic Topaz, Imperial Topaz|
|Crystallography||Orthorhombic. Crystals prismatic, stumpy, sometimes very large, often well formed; also massive, granular, as rolled pebbles.|
|Colors||Colorless, white, gray, pale to medium blue, greenish, yellow, yellow-brown, orange, pale pink, deep pink, tan, beige, red.|
|Density||There is a rough correlation between color and density, as follows: pink: 3.50-3.53; yellow: 3.51-3.54; colorless: 3.56-3.57; blue: 3.56-3.57. The refractive indices and density of topaz have been linearly correlated with the ratio of (OH) to (OH + F) the formula.|
|Cleavage||Perfect basal (1 direction). Fracture conchoidal. Brittle.|
|Stone Sizes||Crystals of topaz may weigh hundreds of pounds and are often quite gemmy at this size. Gems up to 20,000 carats have been cut from material of various colors. Museums seem to delight in obtaining monster sized topaz gems for display. Pink gems over 5 carats (Pakistan) are rare, however, and a Brazilian deep orange gem weighing more than 20 carats is considered large. The largest known pink topaz is an oval of 79+ carats, from the USSR. The largest Brazilian topaz crystal ever found of an orange color (“precious topaz") reportedly measured 5 x 27 cm and weighed nearly 2 kg. A very fine lot (9 cuttable crystals) found in the 1960s weighed over 900 grams and yielded several superb gems, one weighing more than 100 carats and several over 50 carats. The gem giants exist in blue, colorless, and pale yellow colors. Red topaz from the tips of some Brazilian crystals is exceedingly rare, the largest about 70 carats. Smithsonian Institution (Washington, D.C): 7725 (yellow, Brazil); 3273 (blue, Brazil); 2680 (colorless, Brazil); 1469 (yellow-green, Brazil); 1300 (sherry, Brazil); 685 (pale blue, Brazil); 398 (pale blue, USSR); 325 (colorless, Colorado); 170.8 (champagne, Madagascar); 146.4 (pale blue, Texas); 93.6 (orange, Brazil); 50.8 (colorless, Japan); 34 (deep pink, Brazil); 24.4 (blue, New Hampshire); 17 (blue, California). American Museum of Natural History (New York): 71 (red, Brazil); 308 (pale blue, Brazil); 258 (deep blue, Brazil); 1463 (deep blue, egg-shaped, Brazil); 241 (pale orange-brown, Burma). British Museum (Natural History) (London England): 137 pounds (crystal, Norway); 1300 (colorless, Brazil); 614 (blue, Brazil). Royal Ontario Museum (Toronto, Ontario, Canada): 3000 (blue, Brazil); 365 (pale brown. Burma). Los Angeles County Museum (Los Angeles): 1800 grams (orange crystal, Brazil). National Museums of Canada (Ottawa, Ontario): 498.61 (light blue, untreated, Brazil). Private Collection: 173 (blue, Texas); 7,033 (dark blue. treated); 21.327 (light blue, treated, emerald-cut, reputedly the world‘s largest faceted gemstone called The Brazilian Princess): ~79 (pink oval, USSR, world's largest this color but not flawless); 58.8 (pink oval. USSR, flawless).|
|Luminescence||Blue and colorless: weak yellow-green in LW, weaker in SW, greenish white to violet-blue in X-rays, and gems turn brown due to irradiation. Sherry brown and pink: orange-yellow in LW, weaker in SW, sometimes greenish white in SW. This material fluoresces brownish yellow to orange in X-rays.|
|Spectral||Not diagnostic. Heated pink gems contain Cr, and may show a Cr spectrum with a weak line at 6820. As in ruby, this line may reverse and become fluorescent.|
|Enhancements||Pink or Red, may be heat treated. Most blue topaz has been irradiated and heat treated.|
|Formula||Al2SiO4(F,OH)2 + Cr.|
|Pleochroism||Varies with color of material:Dark yellow: citron yellow/honey yellow/straw yellow.Pale blue: bright blue/pale rose/colorless.Dark rose-red: red to dark red/ yellow to honey yellow/ rose red.Rose-pink: yellow/ purple/ lilac.Red-brown: reddish/reddish/ yellow.“Burned "pink: rose/rose/colorless.Brown: yellow-brown/yellow-brown/weak yellow-brown.Green: colorless to blue-green/ green to bright blue—green/ colorless to bright green.|
Topaz (November’s Birthstone)
Inclusions: Usually planes of tiny liquid inclusions, each containing a gas bubble. Some three-phase inclusions have been noted also.
Occurrence: In pegmatites and high temperature quartz veins; also in cavities in granite and rhyolite; in contact zones; in alluvial deposits as pebbles.
New Hampshire: crystals.
Texas: colorless and blue, some facetable to large size.
Pike’s Peak area, Colorado: fine blue crystals in granitic rocks; also colorless, reddish, yellow, some facetable.
Thomas Range, Utah: sherry-colored terminated crystals in rhyolite; facetable.
Minas Gerais, Brazil: fine yellow to orange crystals, facetable to large size: also colorless and pale yellow crystals up to several hundred pounds in size, mostly transparent; pale blue crystals and rolled pebbles, much facetable; some orange crystals contain Cr and when heated (burned) turn pink and show a Cr spectrum. Such material may be distinctly reddish even before heating.
Mardan, Pakistan: fine pink crystals, terminated, cuttable, in limestone matrix, at Ghundao Hill, near Katlang.
San Luis Potosi, Mexico: fine brownish to sherry-colored crystals; also colorless, many excellent forms, cuttable, some yellowish: can be darkened by irradiation but color fades in sunlight.
Urals, USSR: fine blue crystals, often cuttable; also green, magenta colors (gemmy) and pinks from Sanarka.
Jos, Nigeria: fine blue crystals, also white, many cuttable.
Madagascar: various colors in crystals and pebbles, often cuttable.
Sri Lanka and Burma: from the gem gravels, colorless, yellow, and blue gemmy masses.
Queensland and Tasmania, Australia: blue, colorless and brownish gem crystals.
Tingha, New South Wales, Australia: green. gemmy.
Klein Spitzkopje, Namibia: colorless and blue crystals from pegmatites, gemmy.
Zimbabwe; Cornwall, England: Scotland; Japan: crystals and pebbles.
Schneckenstein, Germany: faint yellow, gemmy.
|bluish pale yellow|
|Ouro Preto, Brazil|
|brownish||rich in (OH), Cr|
|Thomas Range, Utah|
|Tarryall Mountains, Colorado|
|Schneckenstein, Saxony Germany|
Types of Topaz Gem Stones
Types of Topaz Gem Stones: Precious Topaz
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!
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.
Types of Topaz Gem Stones: Blue Topaz
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.
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.
Source/Attribution: Dr. Joel Arem