History
The Great Chrysanthemum is a brown diamond discovered in
one of the diamond mines in South Africa, in the year 1963. However the mine
of origin of the diamond is not known. In the rough state the diamond
weighed 198.28 carats and had a light honey color.
The rough stone was purchased by a jeweler from New York,
Julius Cohen, in the same year it was discovered, and taken to New York.
The task of cutting the stone was assigned to the diamond cutting firm
Messrs. S & M Kaufmann of New York, who under the direction of Mr. Cohen
himself, eventually fashioned the stone into a beautiful, rich golden-brown,
pear-shaped faceted diamond, weighing 104.16 carats. The diamond was then
mounted as the centerpiece of a necklace, consisting of marquise, pear, and
round-shaped diamonds.
The Great Chrysanthemum diamond has been exhibited
throughout the United States, at several diamond exhibitions. In the year
1965, the Great Chrysanthemum, was named as a winner of one of the Diamonds
International Awards. Again in the same year, the diamond was exhibited, in
the Diamonds International Academy Collection, at the Diamonds Pavilion in
Johannesburg, South Africa. In 1971, the diamond was exhibited at the
Kimberley Centenary Exhibition held in South Africa.
Julius Cohen later sold the diamond to an anonymous,
reputed, foreign buyer, but presently the owners of the diamond are Garrards
of London.
Chemistry of brown
diamonds
Color in diamonds can be due to three possible reasons :
1 - Presence of impurities in
the crystal structure. Diamonds are crystalline allotropic form
of Carbon. Certain elements like nitrogen, boron, or hydrogen that have
atomic sizes comparable to that of Carbon, can be incorporated in the
crystal structure of diamond, replacing some of the carbon atoms. The
presence of these impurities can impart color to the diamonds by absorbing
light in different regions of the visible spectrum, that brings out
complementary colors to the color that is absorbed. eg. Nitrogen can absorb
light in the blue end of the spectrum, bringing out the yellow color that is
complementary to blue. Likewise nitrogen can sometimes absorb light in
the green region of the spectrum, imparting the complementary color orange.
Boron imparts a blue color to the diamonds. Hydrogen imparts grey, violet
and sometimes green color to diamonds.
2 - Plastic deformation of
the crystal structure. The tetrahedral crystal units of
diamond can sometimes undergo twisting and bending during it's formation or
subsequent rise to the surface from deep inside the earth's crust, a process
that took perhaps 2 to 3 billion years or more. Such deformed areas in the
crystal can alter the absorption spectrum of the stone, imparting pink,
purple, red and brown colors to diamonds.
3 - Exposure to natural
irradiation. Exposure to long periods of irradiation lasting
millions of years, causes "radiation burns", in the form of small patches,
usually only skin deep. These patches are actually areas where the atomic
structure has been altered, which changes the absorption spectrum, imparting
a green color to the diamonds.
The Presence of impurities and deformation of crystal
structure (1 and 2 above) can sometimes occur together giving a combination
of colors eg. Nitrogen impurities together
with plastic deformation can cause color combinations such yellowish brown,
brownish yellow, orangish brown and brownish orange.
Thus brown color in diamonds can be caused by plastic
deformation alone, in which case they are Type IIa diamonds or they can be
caused by plastic deformation in combination with presence of nitrogen, in
which case they are Type 1a or rarely Type 1b diamonds. It is sometimes
stated that brown color in diamonds is caused by stress zones in the
crystal. This is same thing as saying plastic deformation, because it is the
bending and twisting of the crystal units that creates the stress zones in
the giant molecule.
Diamond Classification
Diamonds are classified into two main types depending on
the presence or absence of nitrogen :- Type 1 and Type 11
Type 1 - Contain detectable quantities of nitrogen. 98 %
of all naturally occurring diamonds belong to this group.
Type 11- Nitrogen free or contain undetectable quantities
of nitrogen. Only 1-2 % of naturally occurring diamonds belong to this
group.
Type 1 - Type I diamonds are sub-divided into two :- Type
1a and Type 1b
Type 1a - Nitrogen atoms distributed as groups of two or
more atoms. Groups of two atoms are called A-aggregates, and groups of four
atoms are called B-aggregates. Groups of three atoms are called N3 centers.
Almost 98 % of naturally occurring diamonds are Type 1a. These diamonds can
be colorless or more frequently pale to medium yellow in color, and rarely
orange.
Type 1b - Nitrogen atoms are scattered as single atoms in
the crystal structure. Only 0.1 % of naturally occurring diamonds belong to
this group. The diamonds have an intense yellow or orange colors. Canary
yellow diamonds belong to this group.
Type II diamonds are sub-divided into two :- Type IIa and
Type IIb
Type IIa - Free of all impurities, and usually have
perfectly formed crystals. Thus factors that can cause colors are absent.
Therefore these diamonds are absolutely colorless and are known as the
"purest of the pure" of all diamonds. 1-2 % of all natural diamonds belong
to this group. However some Type IIa diamonds have undergone plastic
deformation, producing colors such as red, purple, pink, brown etc. Their
occurrence is less than 0.1 % of all natural diamonds.
Type IIb - Free of nitrogen impurities but instead
contain trace quantities of boron, which impart a blue color to the
diamonds. Their occurrence is however less than 0.1 % of all natural
diamonds.
Argyle brown diamonds
Argyle diamond mines in Western Australia has an average
annual production of around 35 million carats, which represents about a
third of the global production of natural diamonds. Almost 50 % of this
annual production is industrial diamonds, followed by 45 % of near-gem
quality diamonds. Only 5% of the total production is gem-quality.
Out of the 5 % of gem-quality diamonds produced, 80% are
brown, 16 % yellow, 2 % white, 2 % grey, and less than 1 % fancy colored
diamonds, such as pink, red, and green. Despite the low output of pink and
red diamonds, the Argyle diamond mines is the only consistent source of
these diamonds in the world today.
Most of the Argyle diamonds, whether brown, yellow or
pink are Type Ia diamonds having varying quantities of nitrogen. The
nitrogen content of colorless diamonds are 500 to 1,000 ppm, brown diamonds,
100 to 500 ppm and the pink diamonds, 10 to 100 ppm. In the case of brown
diamonds while the nitrogen imparts a yellow or orange color, the structural
defects impart a brown color, giving a range of colors such as brownish
yellow, yellowish brown, orangish brown and brownish orange, and dark brown
colors.
Argyle Diamond Mining Company has introduced a color
grading scale for their brown diamonds.
Argyle's color grading for brown
diamonds
|
Color |
Argyle Term |
Argyle Color Grade |
|
Very light brown |
light champagne |
C1 - C2 |
|
Medium brown |
medium champagne |
C3- C4 |
|
Brown |
dark champagne |
C5-C6 |
|
Dark brown |
fancy cognac |
C7 |
Most of the diamonds produced in the Argyle mines are
small, and the processing of these diamonds is very labor intensive and
required skilled manpower. The Companies controlling the Argyle mines have
established a partnership with leading diamond cutting firms in India, to
process their large output of small brown diamonds. This partnership is
mutually advantageous to both sides. While the Indian firms were getting
sufficient quantities of rough diamonds to feed their booming diamond
processing industry, the Argyle diamond mining companies, were getting their
diamonds processed at affordable costs, making their industry economically
viable and sustainable. The companies are marketing their products as cognac
and champagne diamonds, and jewelry set with these diamonds have become
increasingly popular, ensuring the economic viability of these mines.
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