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Characestics
Welcome to Twin-Diamonds Gemological Education Center and Guidance
to Gemstones, Diamonds & Jewelry
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Assembled stones
Thin seams of opal are often assembled with backing of opal
or black onyx to produce a doublet, and a clear quartz top
is added to produce a triplet. In addition to making otherwise
unusable material useful, the dark backing enhances the play
of color, and the quartz top adds to durability. Opal doublets
and triplets must still be protected against heat and liquids.
If the adhesive layer begins to break down, the stone's appearance
is marred, and it is difficult, if not impossible, to repair
the damage.
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Heat Conductivity
Heat is conducted differently in various minerals according
to their crystal system. This is used in Thermal Conductivity
instruments to differentiate diamond which
conducts heat very well from its simulants and imitations.
Some instruments use it to identify other gemstones but
they are expensive and of value only when used with care and
some gemmological knowledge. The use of standard stones is
suggested and drafts to be avoided as they can change the readings.
At its simplest this is the temperature test using tongue or
lips for glass and plastic.
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Identification of HPHT-Treated Yellow to Green Diamonds
Examination of recently introduced greenish yellow to yellowish
green HPHT-treated diamonds from
three companies revealed several identifying characteristics.
Gemological properties include a highly saturated body color,
well-defined brown to yellow octahedral graining, moderate
to strong green "transmission" luminescence to visible
light (associated with the graining), and visual evidence of
heating. Most of the treated diamonds, including some from
each of the three sources, exhibit chalky greenish yellow to
yellow-green fluorescence to UV radiation (long- and short-wave).
Distinctive features seen with a handheld spectroscope include
a 415 nm line, a strong band from about 480 to 500 nm, a strong
line at 503 nm, and emission lines at 505 and 515 nm. IR spectra
reveal that they are type Ia diamonds. Near-IR spectra show
a peak at 985 nm, an indicator of high-temperature exposure.
A small number of these HPHT-treated diamonds are
yellow to brownish yellow; they do not show any green transmission,
but other properties identify them as treated.
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New Filling Material for Diamonds from
Oved Diamond Company:
A Preliminary Study
Diamonds filled
with a new glass formulation (XL-21) are being marketed by
the Oved Diamond Company.
These diamonds are
readily identifiable as treated by the intense flash-effect
colors seen with magnification. Durability testing on a small
number of these treated diamonds indicates
that this filler material is more stable to conditions of normal jewelry repair,
such as direct heating with a torch, than the filler material
produced by the Goldman Oved Company a decade ago. Nevertheless,
some damage to the filler was observed in half the new Oved diamonds that
were subjected to a standard prong retipping
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Weather and pollution
How might clouds or pollution affect color? Heavily-polluted
or cloudy skies will result in more grayish (less blue) skylight,
thus improving the appearance of rubies (as
opposed to sapphires).
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Yellow-green to yellow diamonds treated
at high pressure and high temperature (HPHT)
Several companies are now marketing diamonds that
have been HPHT-treated to give them an intense greenish-yellow
to yellow-green color appearance, due in part to green luminescence
to visible light.
Examination of a number of these new treated diamonds revealed
several characteristics similar to those of some naturally-colored
diamonds, as well as a few properties that allow for their
identification.
Features similar to natural-color diamonds included
a saturated bodycolor, well-defined octahedral brown-to-yellow
graining, and moderate to strong green luminescence to visible
light originating from the graining. Infrared spectra for these
treated diamonds revealed
that they are type Ia diamonds with
both A and B nitrogen aggregates and varying amounts of nitrogen.
The highly saturated color found in most of these treated diamonds is
rare in natural-color diamonds; while this observation does
not help identify individual diamonds, it can raise concern
over a parcel of diamonds all
showing strong green-to-yellow coloration.
A more distinctive feature shown by these treated diamonds was
a green-yellow fluorescence to long- and short-wave ultraviolet
radiation, with a "chalky" appearance in both cases.
The visible-range spectrum, observed with a desk-model spectroscope
or recorded with a spectrophotometer, included a weak band
at 415 nanometers (nm), strong bands at 495 and 503 nm, and
bands at 513 and 518 nm (flanked by weak emission bands; the
relative strength of these bands are uncommon in spectra of
naturally colored diamonds).
An important indicative feature was an absorption band in the
near-infrared region at 985 nm. Examination with a gemological
microscope sometimes revealed visual evidence of heating, such
as burned areas on facets, frosted-appearing fractures or portions
of the original crystal surface ("naturals"), and
the presence of numerous, tiny, frosted cracks ("bearding")
along polished girdle facets.
While these gemological features suggest HPHT treatment, there
are also significant similarities to the properties of rare
natural-color diamonds. Careful documentation is required before
a decision can be made whether a particular diamond of
this color has been treated in the laboratory.
For further information on these HPHT color-treated diamonds,
see:
Moses T., Reinitz I. (1999) GIA Gem Trade Lab Notes: Yellow
to yellow-green diamonds treated
by HPHT, from GE and others. Gems & Gemology,
Vol. 35, No. 4, pp. 2023-204.
Reinitz I., Moses T. (1997) GIA Gem Trade Lab Notes: Treated-color
yellow diamonds with
green graining. Gems & Gemology,
Vol. 33, No. 2, p. 136.
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