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Color Enhancements
CIBJO (Confédération Internationale de la Bijouterie, Joaillerie et
Orfèvrerie - also known as the World Jewelry Confederation) and
government agencies like United States Federal Trade Commission (FTC)
ask for the disclosure of diamond treatments performed.
Coloring of a diamond is performed by three
methods:
1. Irradiation
2. Thin Film application
3. High Temperature and High Pressure (HTHP)
The first two are able to just intensify a
diamond color (off colored Cape series stones). Some irradiation methods
produce a thin skin of color, therefore these methods are applied to
already cut and polished diamonds. But HTHP modifies and removes color
and can be performed on both cut and rough diamonds. Only certain
diamonds are treatable by this method. Irradiation and HTHP methods are
almost permanent in effect unlike thin film method. CIBJO demands
disclosure of all color enhancements performed on a diamond.
Irradiation
In 1904, Sir William Crookes discovered radiation’s effects on a
diamond’s color by conducting series of experiments using radium salts,
and he observed that the diamonds enveloped in these salts slowly turned
dark green and the color appeared in patched and did not penetrate deep
into the gem and remained on the surface. This was due to the emission
of alpha (α) particles by radium, but the radioactivity was also somehow
was retained by the diamond, rendering it as un-wearable. The diamond
was donated by Crookes to British Museum in 1914. It has neither lost
its color nor the radioactivity till today.
More safer methods were then devised for irradiating diamonds. These
include proton and deuteron bombardment via cyclotrons, gamma ray
bombardment by exposure to cobalt – 60, neutron bombardment by piles of
nuclear reactors and electron bombardment by Van de Graaff generators.
These irradiation methods are based on moving the carbon atoms from
their place in the crystal lattice and producing a color centre. The
colors thus produced are shades of green, blue or black. Other colors
after further annealing ( defects of irradiation are reduced by
annealing and individual carbon atoms’ mobility increases and the
overall affect depends on the temperature and time of annealing ) of
these diamonds give out bright shades in yellow, orange, brown and pink.
Cyclotroned diamonds have greenish or bluish green color only at their
surface. They are later annealed to obtain a yellowish or orange color.
Their radioactivity remains for just few hours after the treatment due
to the directional nature of the treatment and is imparted only to
selected zones of the diamond. This treatment is now uncommon. Gamma ray
treatment is also becoming unpopular, though it is the safest and
cheapest method, it takes several months for the treatment, and a
blue-green color is produced in whole of the gem.
Irradiation Methods
The two most common methods for irradiation are neutron and electron
bombardments. Neutron Bombardment produces green to black shades penetrating
the whole stone. Electron Bombardment produces blue, blue-green or green
color penetrating just 1 mm of the stone. Further annealing of these stones
(Temperature range being 500–900°C for neutron-bombarded stones and from
500–1200°C for electron-bombarded stones) gives out orange, yellow, brown,
or pink colors. Blue to blue-green gems that are not annealed are separated
from natural stones in the same manner as gamma ray-treated stones.
Before annealing, almost all irradiated diamonds exhibit a
characteristic “absorption spectrum" consisting of a fine range in the
far red, at 741 nm known as the GR1 line. This is usually considered as
a strong indication of treatment. Annealing usually destroys this
property, but generally creates several others. The most persistent of
these resides at 595 nm. However, if the annealing is performed above
1000°C, the 595 line is also destroyed, but leaves two new lines at 1936
and 2024 nm present in the infrared. All these lines can be detected in
gemological laboratories using good quality spectrophotometers. These
lines are best detected when the stone is kept at temperatures below
-150°C.
Some Irradiated Diamonds are Completely Natural
But some irradiated diamonds are completely natural. The famous example is
Dresden Green Diamond. These natural stones get their color imparted by
"radiation burns" in the form of small patches, usually only at the surface,
similar to radium-treated diamonds. Naturally irradiated diamonds also
possess the GR1 line.
Coatings
During the Georgian and Victorian era, colored metal foil coatings were
applied to the pavilion of diamonds as was the first treatment applied
to diamonds after cutting and polishing. These diamonds were placed in
closed-back jewelry settings. Under magnification, there were many flaws
visible such as flaking of foil, air bubbles between the diamond’s
surface and the foil and moisture between the surfaces causing uneven
color. In spite of these flaws, the jewelry consisting of these diamonds
still holds value due to the antique status.
Today, though more sophisticated methods of coating diamonds have been
devised. These include Violet blue dyes and vacuum-sputtered films.
These coatings effectively whiten the apparent color of a yellow tinted
diamond and is usually applied to the pavilion or girdle of a diamond.
These coatings are hard to detect. Dyes can be removed with hot water or
alcohol with ease whereas vacuum sputtered films require sulphuric acid
for their removal. Although under high magnification trapped air bubbles
and scratched layers of coatings are visible. These treatments are
considered fraudulent unless disclosed.
Another method of coating includes application of synthetic diamond
layer on the diamond’s stimulant. This offers resistance to scratches,
higher thermal conductivity and lower electrical conductivity. These
treatments are even more difficult to detect through conventional
methods and are therefore considered fraudulent if attempted to present
a simulated diamond as real.
High-Temperature, High-Pressure (HTHP) Treatment
Few gem-quality stones that possess a brownish body color can be treated
by HTHP method to get their color significantly lightened or altogether
removed. This method was introduced by General Electric in 1999.
Diamonds treated by this method are of Type Iia. The color of these
diamonds results from structural defects that arose during crystal
growth, known as plastic deformations. HTHP treatment is believed to
repair the crystal deformations or according to other researchers, HTHP
destroys stable vacancy clusters and therefore whitens the stone. Type
Ia diamonds having nitrogen impurities present in clusters. These
impurities do not normally affect body color, but can be treated by HTHP
for color alteration. HTHP treatment is also given to some synthetic
diamonds alter their optical properties and make them look like good
quality natural diamonds. Pressure of up to 70,000 atmospheres and
temperature of up to 2,000°C is generally used in HTHP treatment.
Unequivocal identification of HTHP stones is performed by renowned
gemological laboratories. Fourier transform spectroscopy (FTIR) and
Raman spectroscopy are mostly used to analyze the visible and infrared
absorption of suspected diamonds in order to detect characteristic
absorption lines, some of them can be the ones indicative of exposure to
high temperatures. Under the microscope, indicative features observed
are internal graining (Type IIa), partially healed feathers, hazy
appearance, black cracks surrounding inclusions, or a beaded or frosted
girdle. Diamonds that have been treated for color enhancements by
General Electric are given laser inscriptions on their girdle. These
inscriptions can easily be polished and removed. Therefore, HTHP
treatment should be disclosed to the buyer.
Diamond Clarity Enhancements
Diamond enhancements are performed on natural as
well as synthetic diamonds to improve their gemological characteristics.
Enhancements are usually performed on the diamonds already cut and
polished. Enhancements include clarity treatments to remove inclusions,
such as laser drilling, application of sealants for filling cracks,
color treatments for improving a diamond’s color grade or treatments to
give fancy color to a white or off colored diamond. When selling a
diamond....
Read more...
Diamond Fluorescence
Fluorescence is the emission of bluish light emitted by
about one third of gem-quality diamonds when exposed to
long-wave ultraviolet light. The intensity may vary from
faint to very strong. Under normal lighting conditions,
this property of fluorescence is not detectable.
However, a naturally fluorescent diamond tends to emit a
soft colored glow when held under an ultraviolet lamp or
“black light". Fluorescence is not harmful to the
diamond or to the wearer....
Read more..
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Diamond Color Enhancements
