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As long as there have been
gemstones, there have been attempts to improve their appearance. With
the advent of numerous new technologies available to diamond merchants,
those efforts have become increasingly sophisticated and harder to detect.
Most of the treatments discussed in this article have been around for
a decade or more. Others, such as high-pressure high-temperature treatment,
appeared on the market only recently. But no matter what the treatment,
technology doesn't stand still, and even familiar treatments offer new,
improved versions.
The following is a summary of the most common treatment techniques used
on diamonds, how they are identified by trained gemologists, and what
potential problems they present to manufacturers, retailers, and bench
jewelers.
High-pressure, high-temperature (HPHT)
What is it? In this treatment, diamonds
are subjected to high temperatures (1,9000C to 2,1000C/3,4520F
to 3,8120F) and high pressures (6 gigapascals), using diamond
synthesizing equipment. The heat and pressure simulate conditions during
a diamond's formation in the Earth, and can cause alterations in the diamond's
structure and appearance. Whether a color change takes place, and what
the change may be, depends on the diamond that is selected for treatment.
Some diamonds become colorless, while others change to yellow, greenish-yellow,
yellowish-green, blue, or pink. HPHT is used primarily on higher-clarity
diamonds, since inclusions can turn dark or cause fractures during the
treatment process.
Variations. Although some gemological
laboratories have been aware of this treatment since the mid 1990s, the
process was introduced to the jewelry trade in 1999 by General Electric
and Lazare Kaplan International, which created a subsidiary called Pegasus
Overseas Ltd. (now Bellataire Diamonds) to market the treated gems. Since
then, several other companies have begun offering HPHT treatment services
to the trade, including Novatek of Provo, Utah, and Phoenix Crystal Corp.
of Ann Arbor, Michigan. Each company uses its own proprietary process.
How common is it? The HPHT decolorization
process is effective in only a small number of diamonds, making the treatment
relatively rare in the diamond market as a whole. However, among certain
populations of colored diamonds, such as yellow-green diamonds, the percentages
that have been subjected to the HPHT treatment may be significant.
 How
is it identified? Conclusive identification of HPHT treated diamonds
requires low-temperature visible and photoluminescence spectroscopy, techniques
normally available only in gemological laboratories. Some visible signs
that a colored diamond warrants further investigation include high saturation
and darker tones of color; noticeable banded internal graining (in a colorless
diamond); the presence of graphitized "feathers"; and altered
inclusions with surrounding radial fractures. HPHT treated colored diamonds
can also show distinctive fluorescence reactions to long- and short-wave
ultraviolet radiation.
To simplify detection of HPHT treated diamonds, several treaters include
a laser inscription on the girdle of the stone for easy identification.
However, GIA reports seeing several that have no identifying inscription.
In addition, not all treaters are using such identification methods.
Potential problems. The primary concern
with the HPHT treatment is misrepresentation, since treated diamonds can
be difficult to detect. Misrepresentation and/or non-disclosure of these
treated diamonds could also affect natural diamond prices. The HPHT treatment
is considered permanent, and there is no evidence that the treatment makes
the stone any more susceptible to damage during routine repair operations.
New developments: Initially, General Electric and Lazare Kaplan utilized
HPHT treatment to remove color from brown type IIa diamonds, resulting
in colorless or near-colorless gems. Further work has produced certain
type I and type II diamonds in a wide variety of colors, including yellow,
yellow-green, pink, and blue.
Clarity Enhancement
What is it? A high refractive index
glass is introduced into surface-reaching feathers to reduce their visibility.
The treatment is most effective on very thin cracks, since the filling
material is more visible in cavities, laser drill holes, or other gaps.
Variations: Several different companies
offer clarity enhancement services, each using their own proprietary filling
material. However, all filling materials are detected in the same way.
How common is it? First marketed in
the U.S. by the Yehuda Diamond Co. in 1988, clarity enhancement is one
of the more common diamond treatments. Thousands of carats of diamonds
are treated each year by several different companies, including Yehuda,
Oved Diamond Co. in New York City, and Leshem Diamond Co. in Taos, New
Mexico.
How is it identified? A "flash
effect" can be seen when observing the diamond nearly parallel to
the plane of the filled fracture. The flash effect usually appears blue
against a bright background, and yellow or orange against a dark background.
It does not include a full spectrum of color, but is normally just the
two colors. This optical effect can best be seen with 10x magnification
under a focused beam of intense light.
Potential problems. The filler is stable
under normal wear conditions, but it may be damaged by high heat or acids,
which are normally used during repair processes. If the filler is removed
or damaged, the feathers will become more visible. Such damage can be
avoided by removing clarity enhanced diamonds before repair work is done
near the diamond. If damage does occur, many clarity enhancement companies
will re-treat the diamond at no charge.
New developments. In 2000, the Oved
Diamond Co. introduced a clarity enhanced stone they claimed was more
durable and less likely to be damaged if exposed to heat, such as during
repair work. A study of the stones conducted by GIA and published in the
Summer 2000 issue of Gems & Gemology suggested that the new filling
was indeed less vulnerable, but determined it could still be damaged by
direct heat. The authors of the study recommended that all filled stones
continue to be removed prior to repair work to avoid damage.
Laser Drilling
 What
is it? In the traditional method, a laser is used to burn a tiny
channel in the diamond until it reaches a dark inclusion. These inclusions
may be vaporized by the laser, or the resulting channel is used as a conduit
for introducing an acid, which bleaches the inclusion. The resulting white
inclusion is generally considered more acceptable than a dark inclusion.
Variations. Laser drilling is also
sometimes used to provide a conduit to non-surface reaching feathers,
so clarity enhancement techniques can be used.
How common is it? Laser drilling is
one of the more common treatments seen in diamonds. Although GIA estimates
that laser drilled stones represent only a modest percentage of diamonds
in the market, they are common enough that such stones are seen daily
in the GIA Gem Trade Laboratory.
How is it identified? Laser drilling
is normally detected by observing drill holes during microscopic examination
under darkfield, brightfield, and/or fiber-optic illumination.
Potential problems. Laser drilling
is permanent, and normally causes no damage other than the drill hole
itself. However, new techniques that create or extend feathers produce
new structures within the stone, which, like natural feathers, may be
sources of internal tension and breakage.
New developments. In recent years,
new laser techniques have resulted in features that appear as white, irregular,
wormhole-like lines, or larger areas with numerous small feathers in a
step-like progression to the surface, instead of a straight, round drill
hole. In addition, many of these lines are completely internal; only the
feathers reach all the way to the surface. These internal laser drilling
channels are more difficult to see than traditional straight drill holes,
and are more easily mistaken as natural features in the diamond. Microscopic
examination is necessary to identify these laser drilled stones.
Irradiation
What is it? Diamonds are exposed to
a source of radiation to improve or introduce color.
Variations. Artificial irradiation
is typically performed on cut stones, although rough diamonds may also
be irradiated. This treatment (in some cases followed by heat treatment)
is used to produce blue, green, pink, yellow, greenish-yellow, brownish-orange,
and black colors.
How common is it? Artificial irradiation
has been used commercially since the 1950s to produce colored diamonds,
and it's still fairly common. However, HPHT treatment seems to be surpassing
irradiation as the favored color treatment for diamonds.
How is it identified? Some natural
and treated colored diamonds get their color from radiation exposure:
Natural-color diamonds are subjected to irradiation while still in the
Earth, while treated diamonds are irradiated in the laboratory. This makes
conclusively identifying the source of a diamond's color challenging in
some cases. Radiation treatment may produce subtle color zoning and distinctive
ultraviolet fluorescence reactions. Additional information is obtained
from visible and infrared spectroscopy. These tests can only be performed
with sophisticated equipment, however, which is normally only found in
gemological laboratories. As a result, and because natural-color diamonds
frequently sell for premium prices, these stones usually should be sent
to a gem lab for conclusive testing.
Potential problems. Irradiated diamonds
are normally stable to ordinary wear. However, some artificially irradiated
and natural-color diamonds, particularly green diamonds, may change color
if subjected to the heat of a jeweler's torch. This color change is permanent.
As a result, it is recommended that jewelers remove colored diamonds before
undertaking repairs.
New developments. Artificial irradiation
has been used for almost 100 years to produce colored diamonds. Historically,
gemstone irradiation was carried out in a cyclotron or nuclear reactor,
or by using radioactive salts. Today most treatment involves the use of
a linear accelerator.
Coatings
What is it? Diamonds are sometimes
coated with various substances to produce a fancy color, or to make a
pale yellow or brown diamond look more colorless. Coatings are thought
to range from fingernail polish to optical materials similar to those
used on camera lenses.
Variations. The coating may cover the
entire diamond, or only a portion of it. For example, a transparent bluish
coating around the girdle of a very light yellow diamond may make the
stone appear two to four color grades better than its true grade.
How common is it? Coated stones are
rarely seen by gem laboratories, although instances of fraud involving
coated diamonds do occasionally occur.
How is it identified? Coatings are
easily identified by gem labs and by trained gemologists in the field.
Magnification under a variety of lighting conditions, especially diffused
light, will often reveal the coated surface. Coatings may also be scratched
or dissolved in alcohol, which normally would not affect a natural diamond.
Potential problems. Coatings can wear
off during normal wear, so reputable diamond dealers generally avoid this
treatment. As a result, coated diamonds are often seen misrepresented
to unsuspecting buyers as being untreated. New developments. Although gem laboratories
can identify that a diamond has been treated, they normally cannot identify
what the coating is. Therefore, it is difficult to identify when a new
coating is introduced into the market.
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