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. Agrupación Europea de Interés Económico 2006
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.
Related :-
1)
American Golden Topaz.
2)
Famous cut and uncut topaz gemstones.
3)
El-Dorado Topaz.
4)
Lua de Maraba Topaz.
5)
Topaz Amarelo - Yellow Topaz
References :-
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.
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