Origin of name
Azul in the Portuguese language means blue, and the name “Topaz Azul” simply means “Blue Topaz,” an obvious reference to the vivid blue color of the enormous gemstone. Intense blue colors in natural topaz is extremely rare, and the common tone of blue found in nature is pale blue. Thus the “Topaz Azul” with its vivid blue color is obviously an irradiated gemstone, and the Programa Royal Collections, the owners of this massive topaz has stated that the stone has been treated.
Characteristics of the gemstone
Topaz Azul is perhaps the largest faceted treated blue topaz in the world
The “Topaz Azul” is of Brazilian origin, but the weight of the rough stone is not known. The finished stone that had a massive weight of 8,225 carats was a perfect cushion-cut topaz, that was probably colorless or pale blue, two of the colors in which Brazilian Topaz abundantly exist. The clarity of the stone is flawless. The finished stone was subjected to irradiation treatment either by using a linear accelerator or nuclear reactor, or a combination of both, probably followed by heating, to produce the intense blue color of the stone.
The 8,225-carat “Topaz Azul” is the 7th largest topaz in the list of famous topazes greater than 1,000 carats in weight arranged in descending order of weights. The “Topaz Azul” is also perhaps the largest faceted treated blue topaz in the world. Please click here for the list of famous topaz gemstones greater than 1,000 carats in weight.
© Programa Royal Collections.
Why topaz gemstones are ideally suited for color change by irradiation ?
Topaz which is aluminum fluorosilicate having the formula Al2SiO4(F,OH)2 is composed of long chains of aluminum octahedra such as AlO4F2 or AlO4(OH)2, which are cross linked by SiO4 tetraherda. This type of structure can grow into enormous crystals, such as the rough “Topaz Azul”, crystallizing in the orthorhombic crystal system, forming vertically striated prismatic crystals, terminating in pyramidal faces.
Colorless and pale-blue topaz have a slightly higher specific gravity (3.56-3.57) and a slightly lower refractive index (1.61-1.62) than yellow, pink and red varieties, whose specific gravity is between 3.49-3.54 and refractive index between 1.63-1.64. This difference is attributed to the predominance of fluorine (F) over hydroxyl (OH) in the composition of colorless and blue topaz, and vice versa in the composition of yellow, pink and red varieties. Thus colorless and blue topaz seem to be somewhat similar in composition, physical and optical properties.
In topaz, except for the pink and red varieties in which chromium and iron seem to be the color causing agent, all other color varieties including the blue topaz, are caused by color centers, which are an intermediate species formed by trace amounts of iron in the crystal. It is the presence of these color centers that make different varieties of topaz ideally suited for color change by irradiation. Thus colorless topaz can be changed to yellow, brown and blue colors depending on the period of irradiation. The yellow and brown colors may not be stable, but blue color is permanent. Likewise irradiation of naturally yellow, brown or pale blue topaz can produce the intense blue variety of topaz.
History of the “Topaz Azul”
Source of the “Topaz Azul”
The “Topaz Azul” is known to be of Brazilian origin, but it is not known exactly in which state of Brazil, the rough stone was discovered. But, going by the statistics, it can be safely assumed that the source of the “Topaz Azul” was none other than the Minas Gerais State of Brazil, which produces over 80% of the country’s annual gemstone output. Minas Gerais is renowned as the premier source of extraordinarily large crystals of topaz in the entire world, and the “Topaz Azul’ undoubtedly originated in this southeastern state.
Presently, topaz is mined in the so-called topaz-belt, the mountainous region situated west of the city of Ouro Petro, the same area that yielded enormous quantities of gold for almost 200 years, since its discovery in 1693. Two of the mechanized mines that are still in active production are the Capao Mines, situated just outside Ouro Petro, and the Vermelhao Mines, situated about 12 km east of Capao Mines. Several smaller mines that are worked by traditional methods are also operating in the topaz belt. It is not known exactly where the enormous “Topaz Azul” rough stone was discovered.
Irradiation of topaz gemstones to produce the popular blue topazes
Blue topaz is presently the most popular and biggest selling colored gemstone in the U.S. gem and jewelry markets. The main reason for its popularity has been attributed to its low cost, compared to other blue gemstones such as aquamarine and blue sapphire, having a toughness and durability comparable to both these stones, and its abundant availability following a scientifically approved method of irradiating abundantly available colorless and pale blue Brazilian topaz, that is carefully monitored to prevent any hazards of radiation. The common methods of irradiating topaz are as follows :- 1) Use of gamma cells 2) Use of linear accelerators 3) Use of nuclear reactors 4) combination of 2 and 3. It is important to note that irradiation by whatever method is usually followed by heat treatment, in order to remove color centers that interfere with the blue color of the stone. Irradiated stones are also usually left aside from a few weeks to several months, in order to decrease the residual radiation to safe levels.
The blue colors produced by gamma ray treatment are generally lighter in color than the colors produced by the other methods. However there is no characteristic tone of blue that ca be ascribed to any of the other methods, and there is no test that can determine which method of treatment has been applied, and in fact no definitive test to differentiate between treated and untreated stones.
1) Use of gamma cells
Exposure to gamma radiation emanating from cobalt 60 is used in the irradiation of topaz. Such exposure creates both blue and yellow color centers in the gemstones, resulting in a brownish or brownish-green color. The irradiated stones are then heated to remove the less stable yellow color centers, leaving the more stable blue color centers intact. However the color tones produced by this method of irradiation are usually light blue, known as “Cobalt Blue,” which has a lesser demand than the darker blue tones produced by other methods. Prolonged treatment by gamma rays may sometime result in darker blue shades, that are more appropriately called “steel blue” or “grayish blue.” Thus, gamma ray treatment is rarely used for the treatment of topaz today, except as a means of identifying stones that are more susceptible for treatment by other high energy methods available, a process known as pre-treating or screening, which imparts a bluish tint to the stones.
2) Use of Linear Accelerators
In this method, also known as “linac treatment” the topaz gemstones are exposed to a beam of high energy electrons. However the method generates a lot of heat, and the topaz is water-cooled in order to prevent thermal shock, and the spontaneous destruction of color centers. As in the previous method the irradiated stones are heat treated to remove the less stable yellow color centers, leaving a deep blue topaz, with little or no gray overtones, known as “sky blue.” However, the resultant product is radioactive, and should be left aside for several weeks until the residual radiation reduces to safe levels. Close monitoring of the process is very important, before the treated stones are released to the markets.
3) Use of Nuclear Reactors
In this method the topaz gemstones are exposed to fast moving neutrons, which creates only blue color centers, and therefore does not require subsequent heating. The color produced is a medium to dark grayish-blue known as “steely” or “inky” and referred to in the trade as “London Blue.” The “inky” tone can be reduced to some extent by heat treatment. However, topaz treated in this way tend to be quite radioactive and may have to be put aside for a longer period than the previous methods, before radioactivity decays to safe levels. The Nuclear Regulatory Commission monitors and regulates the whole process, and all producers and importers of treated topaz are licensed by this commission.
4) Treatment combining 2 and 3 above
The last method of irradiation combines the use of nuclear reactor and linear accelerator for various periods of time, and is subsequently followed by heating, to give a product that has dark blue tones without the inkiness of the “London Blue” material. The blue product here is known variously as “electra blue,” “Swiss blue” etc.
The method of treatment applied for the “Topaz Azul”
In view of the fact that there is no characteristic tone of blue that can be ascribed to a particular treatment method, except perhaps the first method which produces lighter tones of blue, known as “Cobalt Blue,” it is difficult to say which one of the remaining three methods have been applied in the treatment of the “Topaz Azul,” but the lack of grayishness of the “Sky Blue” and the “inkiness” of the “London Blue” seem to suggest that the blue color of “Topaz Azul” is actually a “Swiss Blue” produced by the 4th method, which combines the use of the the nuclear reactor and the linear accelerator, followed by heat treatment.
“Topaz Azul” enters the Programa Royal Collections
The treated “Topaz Azul” gemstone was acquired by the Programa Royal Collections after it was set up in the year 1997, by an European Economic Interest Grouping, regulated by the European Union. The enormous gemstone was placed under a group of 20 famous gemstones, known as the Special Exhibitions Collection, brought together for various reasons such as extraordinary size, exceptional quality, historical value, rarity of the gemstone etc. The “Topaz Azul” occupies the 4th position in this collection when arranged according to descending order of carat weights. Please click here for the table of Special Exhibitions Collection arranged according to descending order of carat weights, given on the web page devoted to “Topaz Amarelo.”
While “Topaz Azul” remains a permanent part of the Special Exhibitions Collection, it is sometimes moved to the single theme Monographic Exhibition on topaz, known as “Imperial & Noble” which consists of a collection of 54 topazes of all sizes, cuts and colors, having a total weight of 90,000 carats, one of the most comprehensive topaz collections in the world.
For more information on the Programa Royal Collections such as the objectives of the collection, categories in the collection, educational value of the collection, categories of gem collections, etc. please refer to the page on the “El-Dorado” Topaz.
You are welcome to discuss this post/related topics with Dr Shihaan and other experts from around the world in our FORUMS (forums.internetstones.com)
1) Web article on Topaz – Department of Geological Sciences, University of Texas.
2) Gem & Crystal Treasures – Peter Bancroft, (1984)
3) Web page on Topaz – International Colored Stone Association
4) The Mineral Topaz – Galleries .COM
5) Topaz- Mindat .ORG
6) Website of Programa Royal Collections – Madrid, Spain.