The Complete Identification Guide
of Real Pearls
Diamonds may be a girl's best
friend, but thousands of women still lust for the timeless elegance and glamour
of pearls.
Knowing if a pearl was
produced by an oyster or by a machine in a factory is critical in determining
value. Below are some steps you can take to see if a pearl is real or
fake. Ultimately, a graduate gemologist from Gemological Institute of America (GIA) is the best source for
determining a pearl authenticity and Aucoin
Hart has several on staff to assist with determining a pearls authenticity.
Identification of a pearl, or
pearls, seldom requires use of conventional gemological testing techniques.
Instead, the gemologist must use a combination of observed visual features, and
some specialized techniques of identification such as direct X-radiography, and
if available X-ray diffraction, to identify the type of pearl.
Recognition of value
enhancement in pearls is of increasing significance to gemologists. While some
treatments (gemstone
treatment), such as: pearl ‘skinning’, routine color enhancement of drilled akoya cultured pearls by bleaching
and dyeing, bleaching of pearls to produce uniformly white pearls, and surface waxing to enhance the luster of
pearls, are considered to be acceptable trade practices, other value
enhancements of pearls, such as dyeing and/or irradiation of pearls to enhance
their color, now need to be recognized and disclosed.
Faux Pearls: Types of Imitation Pearl
Common Imitation
Pearls
Imitation or simulated pearls
have been around for centuries, long before the existence of cultured pearls. Plastic, glass, and shells are
used to create these pearls, and a substance made of fish scales is often
applied to the surface, giving the pearls a lustrous, nacre like appearance. Majorica pearls are simulated pearls
that are often mistaken for cultured pearls. Majorica strands range in price
from twenty-five to seventy-five dollars in the better department stores. To
tell the difference between these imposters and cultured pearls, rub the
suspect pearl against the bottom edge of your upper teeth; cultured and natural
pearls have a coarse, gritty feel, whereas simulated and imitation pearls are
smooth.
These
imitation pearls are usually coated with something to give them a pearly
appearance, such as pearl essence, powdered mother-of-pearl and synthetic
resin, synthetic pearl essence, plastic, cellulose,
and lacquer.
1) Glass
Beads
The beads are cover with as
many as forty coats of pearl essence and hand polished between each coat.
2) Plastic
Beads
Make in the same process as
that of glass beads, but is much lighter than glass beads.
3)
Mother-of-Pearl Shell Beads
These are coated with the same
substances as plastic and glass imitations. There are also some occasions that
people sell uncoated mother-of-pearl shell beads, and claim those are pearls.
Other terms used to designate
imitation pearls are simulated and faux pearls. So how can you tell if a pearl
is real? Here are a few tips that can help you identify real pearls from the
fake ones.
How Do you Test if Pearls are Real?: Practical Methods
1) The ‘Tooth Test’
In spite of what some non-gemologist
jewelers believe, an ‘educated eyeball’ and the gritty feel of nacre on teeth (due to overlapping platelets of
nacre covering the external surface of a pearl) will not discriminate natural
from cultured pearls. However, to the pearl expert who does not wear dentures,
the absence of ‘grittiness’ could suggest one’s teeth are examining an
imitation pearl.
2) The Sun Test
This is my favorite test. It
involves taking your pearls out into the sun or holding them under very bright
indoor lighting. Unless they are very expensive, genuine pearls won't be perfectly matched under the sun.
You will be able to see variations in
their iridescence (orient) and color. If the pearls are perfectly matched
for color and overtones, they are most probably fake. If you are buying
pearls from a seller who offers
pearls that are perfectly matched, the cost of a gemologist certificate (for a
gemologist of your choice, not his) is a minimal part of the investment. It
costs about $150 to have pearls tested, as opposed to several thousands of dollars
for the type of pearls that warrant the test.
3) Friction
Test
Take two pearls then lightly
rub one against the other. If they feel gritty or sandy, they are real pearls.
If they feel smooth, they are not real.
4) Magnification Test
As is true with diamonds,
magnification reveals a lot about the quality of a pearl. You can see the characteristic ridges and irregularities of real pearls or the
grainy smoothness of fakes. You can examine drill holes to see the interface
between the nacre and what lies beneath it. You can read any writing on the
clasp or setting.
5) Density Test
Density is the mass of an
object as a function of its volume. Real
pearls are heavier for their size than plastic, resin, or hollow glass
pearls. Good glass fakes will have the same density are real pearls. Light
pearls are fake - you can't tell real from faux on the basis of density alone
if the pearls are heavy.
6) Temperature
Feel
The first step you can take is
to touch them and feel the temperature. Real
pearls are cold to touch for the first couple of seconds before warming up
against your skin. Fake plastic pearls have the same temperature as the room
temperature and you don’t feel the coolness when you touch them. However, fake
ones that are made of glass beads
can be cool to touch to start with. But it tends to take them longer to warm up
against your skin than real pearls. In addition, genuine pearls tend to warm to
the skin much faster than glass pearls. Resin or plastic pearls tend to feel
somewhat warm upon first contact.
7) The Drill Holes
Real pearls tend to be drilled
from both sides, to meet in the center. If you could see the cross section of
the pearl, the hole may appear wider at the outside edge of the pearl than at
the center (which can make stringing poorly-drilled pearls very challenging and
is one reason many people won't restring pearls that they didn't sell). Holes of real pearls tend to be as small as
possible (with some exceptions), since the weight of a pearl affects its
price (more hole means less weight and lower pearl
value). Inexpensive real pearls may
be lower in cost because the drill holes are not completely straight. Fake
pearls often have larger, possibly straighter holes than real pearls. Some
fakes are made to have smaller holes, so that they can be knotted like their
genuine counterparts. Inexpensive fakes may have holes of widely variable sizes
on a single strand.
The nacre of fake pearls is
more likely to flake away near the drill
hole than on a cultured pearl (it
won't flake on a natural pearl). Either the flaking or the sight of a clear
inner bead may clue you in to a fake. Most fakes have pearl-colored centers, so
the center color may not help you. The holes of fake pearls often form a
shallow bowl shape, while the holes of real pearls are more likely to be flat.
Examining the hole is also a good way to detect signing of dyeing.
8) Destruction
If you cut a pearl open, you
will see its true nature. Natural pearls are comprised of layer upon layer of nacre. Cultured pearls have a shell
(mother-of-pearl) core covered with a thin layer of nacre (generally no more
than half a millimeter, usually much thinner). Fake pearls have a core with one
or more layers of coating applied to them, which tends to flake away from the
core upon cutting. Cutting a pearl reveals the nature of its drill hole, if
present. Of course, you need to be able to tell pearl-colored glass from shell
in order to do this test (plastic and resin are easier to discern). Also,
you'll destroy the pearl. It isn't recommended.
9) Other Visual Clues
Fakes tend to look 'flat' in comparison to the real
thing. There are exceptions, of course, with beautiful simulated pearls made by Swarovski and other
manufacturers. Real pearls tend not to be perfect and may have bands in their
nacre, bumps, ridges, or pits. They vary in size and shape from one to another.
Genuine pearls may have concentric ridged circles around them, which
inexperienced people may take for marks from molding of a fake (which is seen in the exact middle of all the
pearls on strands of some faux pearls). Real pearls come in many shapes,
but they tend not to be perfectly round, so a perfect sphere should be suspect. Expensive genuine pearls may be
round, but you will have other clues to help you make a determination. Some fakes are made to look irregular, and
glass pearls often have flattened ends or slightly oval shapes.
10) Recommended More Technical or Scientific Identification Pearls Protocol
(including identification of natural pearls from cultured pearls)
To technically identify an
individual pearl or a strand of pearls, the following steps should be followed:
Step 1. A general examination of the pearl(s)
Step 2. A detailed examination of the pearl(s)
external surface(s), drilled hole(s), and transillumination of the pearl(s)
Step 3. Direct X-radiography
Step 4. Other laboratory-based tests that could
include X-ray luminescence, cathodoluminescence, specialized
spectroscopy such as UV-visible, X-ray fluorescence, Fourier IR and FTIR, Raman spectroscopies,
X-ray diffraction and perhaps
examination with an endoscope if a working instrument is still in existance
Step 5. Detection of value enhancement.
Step 1: General Examination
(Natural vs. Cultured Pearls)
Strands of pearls should be
examined initially by first holding the strand taunt between both hands, and
then examining the necklace as a whole against a neutral colored background of
light grey or matt white colour using fluorescent white overhead illumination
from an articulated double bar desk lamp. Similar conditions of lighting should
be used when examining a single pearl. Use of low magnification, such as a 4
head loupe, may assist this examination.
The following observations can
be used to assist the discrimination between natural and cultured pearls.
a) Due to the rarity of natural pearls of
uniform size and shape, strands of natural pearls are almost always graduated.
So, if a strand consists of spherical pearls of very uniform size and shape, it
likely contains bead nucleated cultured pearls.
b) For the same reason, uniformity of colour
matching usually will be much better in a cultured pearl or imitation pearl
necklace than in a natural pearl necklace.
This observation can often be
confirmed when strands of pearls are examined under LWUV. While strands of bleached
akoyas, white South Sea pearls and white freshwater pearls usually display a uniformly
strong milky bluish white fluorescence, strands of natural pearls usually
contain pearls that fluoresce with variable intensities, often in yellowish to
greenish to tan colors. Strands of natural colored black pearls commonly will
display a brownish to reddish LWUV fluorescence of variable intensity, while
strands of dyed and/or irradiated cultured pearls commonly are inert to LWUV,
or may display fluorescence to LWUV of different colour but will be of relatively
uniform intensity.
c) ‘Circles’
and ‘fish tails’ are characteristic
shapes of bead nucleated cultured pearls.
d) If a strand of pearls or a single pearl
‘blinks’, when rotated and transilluminated at arm’s length, the presence of a
bead within the pearl should be suspected.
e) Usually, cultured bead nucleated South Sea
pearls, recent Chinese round freshwater pearls, bead nucleated cultured
freshwater pearls from lake Kasumiga in Japan and rare natural pearls are the
only round pearls likely to have diameters in excess of 8 mm.
Step 2: Detailed Visual Examination
First, the external surface of
pearl(s), and any drilled channels in those pearl(s), must be thoroughly
cleaned of adherent debris and grease by washing the pearls in luke-warm soapy
water with a soft cloth and soft bristle toothbrush. Once clean and dry, the
identification of most pearls can be simply accomplished with the 10 hand lens
and/or low power binocular microscope and good fibre-optic illumination, by examining:
The Nacre of the Pearl, noting
the presence of:
a) A thumb
print-like pattern due of serrations caused by the edges of overlapping
lamellae of platelets of aragonitic nacre intersecting the external surface of
both natural and cultured nacreous pearls. This hand lens appearance contrasts
strongly with the smooth ‘blotting paper’ texture of the essence de orient coating on imitation pearls, the frequent
loss of part of this coating from underlying glass, plastic or shell beads that
form the base of these imitations, or the smooth glassy surface of a hollow
glass sphere coated internally with essence d’orient.
The examination of the surface
of nacre at higher magnifications can prove useful for discriminating between
the nacre of Chinese tissue grafted pearls and natural pearls, for the
freshwater cultured pearls display finely granular nacre that is covered with
fine parallel ridges that run straight across the surface of the nacre.
b) The smooth, often scratched worked surface
of a polished shell bead imitation pearl. Transillumination usually readily
reveals the parallel growth banding of the shell from which the bead imitation
was manufactured, irrespective of the color of that shell.
c) Accumulations of dye on the surface, or
under the surface of dyed mantle grafted freshwater pearls and akoya bead nucleated saltwater cultured pearls.
The porcellanous
‘flame’-patterned surface of a non-nacreous pearl, as exemplified by the subtle
‘flames’ that decorate the external surface of white, pink, orange and brownish
conch pearls.
d) Distinctive planes of junction on natural
blister pearls, cultured half or three quarter pearls, mabé pearls and the
composite imitations known as coque-de-perles and Osmeña pearls.
Secondly, the drill hole
should be examined with either: transmitted illumination of variable intensity
and magnification from a 10 hand lens
or a low power binocular microscope;
incident illumination and magnification from a 10 hand lens
or a low power binocular microscope; or transilluminated miniature fibre-optic
illumination of variable intensity and magnification from a 10 hand lens
or a low power binocular microscope, noting:
e) The concentric, thin ‘onion’-like
alternating layers of aragonite and dark-colored
conchin that form a natural pearl.
Note, however, that the color of the walls of the drill hole in a natural pearl
commonly grades downwards from yellowish (peripherally) to a darker brown towards
the centre of the pearl.
The presence of a greyish or white central shell bead, and thin layer of brownish or other colored
organic conchin between this bead and its surrounding circumferential layer of
nacre, in a bead nucleated cultured pearl.
With respect to this
observation, it should be remembered that as bead nucleated cultured akoya pearls are routinely bleached and dyed, the thin layer of
conchin between the bead and nacre of this pearl is usually dyed the same
colour as that displayed by the body colour of the pearl. Colored South Sea
pearls also should be checked for this feature, for although these cultured
pearls used to be dyed (only rarely), the frequency of this treatment is
increasing.
The observed thickness of
nacre also can be used to provide some indication of the type of pearl the gemologist
could be examining, as the thickness of nacre surrounding the bead of a South
Sea pearl is 1 mm, while 0.5 mm of nacre usually surrounds the
beads of most akoya pearls of modern production.
The stringing channels of
black pearls require careful examination, for the pearl being examined could be
either a natural colored black pearls, a natural colored black keshi pearl, a bead nucleated black South Sea cultured pearl, a
black dyed bead nucleated cultured pearl, a black and/or irradiated dyed mantle
grafted freshwater cultured pearls or an irradiated bead nucleated or
non-nucleated cultured pearls.
These black pearls can be discriminated
with relative ease, by careful examination of the pearl’s drilled stringing channel, and noting that:
1) Natural black pearls will have an ‘onion-like’ structure of alternating thin
darkly colored lamellae of aragonite and conchin.
2) Natural black keshis will have a similar
structure to natural black pearls, but an irregular
cavity may be detected in the centre of the pearl.
3) Bead nucleated cultured pearls have a white
to greyish MOP bead, usually a thin layer of black conchin, and a
circumferential black nacre of up to 1 mm thickness.
4) Black dyed bead nucleated cultured pearls
will have a dyed black bead and black nacre. Black dye may also be found
adhering to the surface of stringing channels in the pearls. Sometimes the
black dye also stains the thread on which the black dyed pearls have been
strung.
5) Black-dyed mantle-grafted freshwater
cultured pearls may have black dye adhering to surface defects, and in the
drill hole and staining the thread.
6) Irradiated black bead nucleated pearls are
characterized by having a dark-colored bead and lighter-colored white to
greyish nacre surrounding that bead.
7) Very iridescent irradiated black freshwater
pearls will display an external surface on which the thumb-print pattern of
nacre loses its definition.
8) The partial absence of the usually thin
layer of artificial essence d’orient that
surrounds the stringing channel of the glass (solid or hollow), plastic or
shell bead of an essence
d’orient coated imitation pearl. In
addition, glassy conchoidal fractures may be observed surrounding the stringing
channel of glass imitation pearls, while the periphery of the stringing channel
of plastic imitation pearls will often display ‘mould marks’ and will peel when
judiciously pared with a small, very sharp blade.
9) The presence of a detectible central cavity
in the wrinkled nacre surrounding the stringing channel of mantle grafted
cultured freshwater pearl that are often known as a Biwa or Chinese ‘rice
crispie’ pearls.
10) The presence of two or three alternating
growth layers in a round 8 mm Chinese mantle grafted cultured
freshwater pearl.
Thirdly, Transilluminate
the pearl:
transillumination of a pearl, centrally located over an adjustable diaphragm,
can usually reveal the striped (layered) structure of the shell bead that is
centrally located in a bead nucleated saltwater cultured pearl, or the vague
shadow cast of a large irregular central cavity in a non-nucleated (freshwater)
cultured pearl. Remember, however, due to the absorption characteristics of
black nacre and dyed black shell beads, transillumination will prove to be of
little use for examining bead nucleated
black pearls.
Step 3: Use of Direct X-radiography
Few working gemologists will
have access to facilities for X-radiographing pearls to determine their
identity; for by law X-radiography is a technique of identification only
available in well-equipped large
research or gem testing laboratories that have suitably qualified staff.
Therefore most working gemologists will have to rely on a detailed visual examination,
as specified in steps 1, 2 and perhaps refer the pearl(s) to a laboratory to
provide answers to questions such as:
Is the
pearl natural, cultured or imitation?
If the
pearl is cultured, what type is it?
Those gemologists and/or
scientists who are legally qualified and licensed to use X-rays for diagnostic
or research purposes must have access to at least a 10 mA, 70–90 kV X-ray unit that produces a collimated beam of
X-rays; a dark room equipped with controlled development and fixing facilities
for X-rays; and a supply of fine grain X-ray film that will record a pearl’s
internal structure with maximum definition.
Lead foil masks, or immersion
in a non-staining contrast medium should be used to minimize fogging of the
developed film by scatter of X-rays by the pearl. Pearl(s) should be examined
with a bracket of exposures ranging from under-exposure to over-exposure to
ensure that both the outer and inner structures of the pearl are recorded in
diagnostic detail on the film. Each pearl should be radiographed in at least two predetermined orientations at
right angles to each other. Exposed X-rays must be developed and fixed to
the manufacturer’s precise specifications, and importantly all processed
radiographs should be examined dry, with 5–10 magnification, with X-ray viewers
equipped with transmitted white light of variable intensity.
Direct X-radiography has two
uses in pearl identification. First, direct X-radiography can be used to
provide a general survey (overall impression) of the composition of a pearl
necklace – particularly if the pearls have been strung in such a manner that the
stringing holes are not accessible to visual examination. Secondly, to ensure
an accurate identification of each pearl in a necklace the pearls must be
unstrung, and individual X-radiographs used to determine the identifying
radiographic structures of each pearl. Also, individual X-radiographs should be
used to identify any pearl that is undrilled, pegged or set into jewellery.
When pearls are examined by
direct X-radiographs, the resulting negative images on the films (white for
calcified tissues and black for soft tissues) do provide a permanent record of
the identifying structural features of the majority of pearls. For example:
a) Natural pearls display thin arcs and rings of radiolucent black conchin
within the white X-radiopaque image that represents a pearl’s nacre. Sometimes
natural pearls display a small central X-radiopaque cavity.
b) Keshi pearls have the same structure as natural pearls,
but they may display a central radiolucent cavity of irregular outline and of variable
size. Bead nucleated cultured pearls display, from the inside out, a dense white
structureless central X-radiopaque bead, a thin to thick black circumferential
layer of radiolucent conchin, and comparatively thin (for the akoya pearl) to thick (for the South Sea pearl) slightly less
X-radiopaque external layer of nacre.
Non-round mantle grafted Biwa-type cultured pearls usually
display a relatively large central radiolucent cavity of quite variable size
and dimensions.
c) Rounded Chinese freshwater cultured pearls
may contain a rather flattened central
X-radiolucent cavity of variable size and dimensions if they have been
mantle grafted. In contrast, if the pearls have been bead nucleated they will
display the same X-radiographic features as those of the akoya and South Sea pearls.
Occasionally the X-radiograph of
rounded Chinese freshwater pearls will indicate that they have been formed from
a radiopaque central off-round possibly pearl nucleus, a circumferential layer
of X-radiolucent conchin and an external layer of X-radiopaque nacre.
d) Cultured
half-pearls are shown to consist of a thin X-radiopaque hemisphere of
nacre, a flat X-radiopaque base of polished shell, and cavity filling of
adhesive polymer that is usually X-radiolucent.
e) Imitation pearls display a variety of
appearances on a X-radiograph. Polymer bead imitations give no image as the material is
X-radiolucent. In contrast, solid glass beads and shell beads provide a dense white structureless image. Hollow
glass beads have a characteristic image that consists of a thin X-radiopaque
rim surrounding a large X-radiolucent central cavity.
In summary of this step in the
identification of pearls it must be stressed that direct X-radiography will not necessarily reveal the identity of some
pearls, such as South Sea pearls
with thick layers of nacre, or natural
pearls that display no X-radiolucent arcs or central concentrations of
radiolucent conchin. Also, the interpretation of identifying features recorded
on direct X-radiographs of pearls is very much a learned and practiced skill.
Step 4: Other Scientific Tests for Pearls
Those pearls that cannot be
identified either by observation of identifying features or by direct
radiography should be submitted to a recognized pearl identification laboratory
for more sophisticated testing by, for example, X-ray fluorescence analysis, cathodoluminescence,
X-ray diffraction, microchemical analysis by electron
microprobe, Raman spectroscopy or,
perhaps, endoscopy.
1) X-ray Luminescence of Pearls
In the laboratory, whether a
pearl has a saltwater or freshwater origin can be confirmed or denied by
testing the pearl’s luminescence to high
voltage X-radiation.
When the shell bead of a
cultured pearl is irradiated with X-rays it will fluoresce, and then
phosphoresce a distinctive yellow-green colour. This X-ray luminescence
response is typical of the freshwater mussel shell from which the beads of
cultured pearls have been fashioned. This characteristic luminescence is due to
a high concentration of manganese (Mn2)
in freshwater shell.
For the same reason, natural
freshwater pearls, and undyed and unirradiated mantle grafted freshwater
cultured pearls also will fluoresce and phosphoresce a similar bright
yellow-green colour under X-ray excitation. In contrast, as natural saltwater
pearls, and the nacreous ‘skin’ of pearls cultured in a saltwater marine
environment have a relatively low manganese content, this renders these pearls
inert or only very weakly fluorescent to X-radiation.
Unfortunately, the precise
identity of a pearl, which fluoresces and phosphoresces a garish bright
yellow-green colour under X-ray, must remain in doubt since the pearl could be
either a natural freshwater pearl, a tissue grafted freshwater cultured pearl
or a bead nucleated cultured pearl with relatively thin nacre. Fortunately, on
a bead nucleated cultured pearl, in which the X-ray induced luminescence has to
shine through the layer of non-fluorescing nacre that surrounds the shell bead
of the pearl, this subdues the intensity of colour observed by an amount
proportional to the thickness of its nacre. Therefore, if a pearl in question
fluoresces a yellow-green colour of medium intensity, it could be either a
natural pearl or bead nucleated cultured pearl.
However, according to an
expert in pearl identification from Australia, Kennedy (1998), if a
yellow-green phosphorescence follows the fluorescence, this indicates that the
pearl is more likely to be a bead nucleated cultured pearl. Confirmation that
the luminescence is emanating from the shell nucleus can be obtained by angling
the drill channel to face the window of the X-ray machine. Then, the
yellow-green colour will be observed to be more intense within the drill
channel. However, if the ‘skin’ of nacre of a nucleated cultured pearl is thick
enough, it is possible that no fluorescence let alone phosphorescence will be
observed.
2) Cathodoluminescence
Cathodoluminescence (CL),
luminescence emitted in response to bombardment with high-speed electrons, can
be used to discriminate saltwater from
freshwater pearls, and also to discriminate
natural freshwater pearls from Chinese tissue grafted cultured pearls. This
discrimination is based on the presence or absence of manganese Mn2 in the
nacre of the pearls, and differences in luminescent intensity of natural
freshwater and Chinese tissue grafted cultured pearls.
Based in scientific findings
by Banerjee and Habermann (2000) have revealed that Mn2 activated
CL spectra of freshwater pearls and shell is characterized by a strong peak at
566 nm (in the green) and a weaker peak at 420 nm (in the blue) related to
biological aragonite and a peak at 640 nm (in the orange) attributed to biological calcite. Non destructively
irradiation of a pearl’s surface in a cold cathode CL-microscope will reveal
that the intensity of 566 nm CL is much higher in freshwater pearls than tissue
grafted freshwater cultured pearls due to the basic fact that the surface of
natural freshwater pearls contain more Mn2 than the surface of freshwater cultured
pearls. Saltwater pearls do not contain Mn2 and do
not luminesce when exposed to CL.
3) UV-VIS Spectroscopy
A study by Elen (2001, 2002)
has revealed that the natural colour of pearls can be confirmed by the combined
use of UV-VIS (ultravioletvisible) reflectance
spectroscopy and LWUV fluorescence.
With respect to natural colored pearls derived from the nacre of the
black-lipped pearl ‘oyster’ P.
margaritifera:
its white
nacre displayed a strong light yellowish LWUV fluorescence, and no identifying
absorption features; its black to grey nacre displayed a most identifying
absorption at 700 nm, other absorptions at 495 and 405 nm, and a LWUV fluorescence
that ranged from reddish to reddish brown to brownish of weak to moderate
intensity; its yellow nacre could display the 700 nm absorption, often
accompanied by a 495 nm absorption that identifies P.
margaritifera nacre.
Elen further recommended that
if visible absorptions are not present, the observed presence of an absorption
feature between 330 and 385 nm in UV, accompanied by a light yellow/greenish
yellow/ greenish brown/light brown LWUV fluorescence will identify P. margaritifera as the source of the nacre.
With respect to treated black
nacre it is important to remember that this usually does not fluoresce when
exposed to LWUV. In contrast, with respect to pearls from the nacre of the
silver-lipped ‘pearl oyster’ P.
maxima:
its white
nacre from P. maxima generally displayed a moderate
to strong light blue to light yellow LWUV fluorescence and no identifying
absorption features.
However, its yellow to golden
nacre displayed a characteristic increase in absorption between 330 and 385 nm as the tone of saturation of the yellow to
golden color increased. This was associated with an even distribution of body
color and a LWUV fluorescence of moderate intensity that changed from light blue/light
yellow to light brown, greenish yellow, greenish brown or brown as the colour
of the nacre darkened.
In contrast, treated (possibly
heated) yellow to golden nacre did not display the 330–385 nm absorption of
yellow P. maxima nacre, and its LWUV fluorescence tended to be
patchy, in spite of the fact that the distribution of treated colour could be
either even or display small spots of concentrated colour.
4) X-ray
Diffraction of Pearls
If serious doubts exist about
whether a bead is present in a pearl, then the X-ray diffraction technique (or
more specifically the lauegram method)
can be used to identify whether the pearl is natural or a bead nucleated cultured
pearl. This identification is based on differentiating the X-ray diffracting
properties between the layered structure of the shell bead (theoretically thick
aragonitic nacre from the Mississippi River mussel) within a bead nucleated
cultured pearl, and the concentric symmetrical aragonitic nacreous structure of
a natural pearl.
In practice, all X-ray
diffraction patterns obtained on the negative X-ray film have a large central circle
‘blackened’ by the X-rays passing centrally through the pearl. This black spot,
in theory, should be surrounded by recognizable, distinctive patterns of spots
created by the X-rays that are diffracted further outwards by the crystal
structure of the pearl. If a very fine collimated beam of X-rays is passed
through the precise centre of a bead nucleated cultured pearl, perpendicular
(at right angles) to the layers of its shell bead, the X-rays will encounter crystals with the same symmetry as
displayed by a natural pearl.
The pattern obtained on the
film which will vary from a hexagonal outline to a hexagonal arrangement of spots is caused by the
hexagonal symmetry down the c-axis of the aragonite platelets that form
nacre. The major spots are situated at apices of the pattern, giving rise to
the description of the pattern as a ‘6-spot pattern’. If a second lauegram is
taken at right angles to the first direction, the X-rays will now be travelling
parallel to the layers of the shell bead. A different crystal symmetry,
perpendicular to the principal axis of the aragonite platelets, will be
encountered. This results in a squarer or more rectangular pattern with four
internal spots within the outline that reflect the 2-fold symmetry of the aragonite
crystal structure in this direction. Hence the description of these being ‘4-spot patterns’.
In contrast, if the pearl is
natural, an identical ‘6-spot pattern’
would have been obtained at right angles to the orientation at which the first ‘6-spot
pattern’ was obtained. Simply put, two hexagonal diffraction patterns taken
from directions perpendicular to each other, in the absence of any contrary
evidence, indicates that the pearl is natural. With any pearl, great care must
be taken to ensure that the X-ray beam passes precisely through the centre of
growth of the pearl. The reason for this precision is that in thick-skinned
bead nucleated cultured pearls a hexagonal pattern also may be obtained in both
directions. The reason for this is that the thick nacreous ‘skin’ of this cultured pearl produces a anomalous ‘6-spot
pattern’. However, if the direction of the X-ray beam can be made to
precisely parallel the layers of its bead nucleus, a discriminating superimposed
‘4-spot’ pattern should possibly be discernible within the overall hexagonal
pattern.
5) Using Endoscope
The endoscope was an
instrument that was invented, in late 1926, to facilitate examination of the
stringing channels in drilled pearls. The endoscope was designed to facilitate
discrimination of natural pearls from the then newly marketed Japanese bead
nucleated cultured akoya pearls. Although this instrument is very
rarely used today, its principle of operation should be understood by gemologists.
The endoscope utilizes a very
thin hollow metallic needle to penetrate the stringing hole of a pearl, so that
a strong narrow beam of white light that is delivered through the needle to a
45° pyramidal mirror located at its end is reflected vertically into the pearl
being examined. As the needle of the endoscope is inserted into the stringing channel
of the pearl and directed towards the centre of the pearl, the reflected narrow
beam of light will be either transmitted around a hemispherical layers of nacre
in a natural pearl and returned into the stringing channel, so that a flash of
light can be viewed in an eyepiece located opposite the inserted needle; or,
the beam of light will be transmitted to the surface of the pearl along the
flat layers that form the bead in a bead nucleated cultured pearl. In this
situation, an identifying spot of light then will be observed on the external
surface of that cultured pearl.
With respect to the use of the
endoscope, it is important to remember that as natural pearls, keshis and tissue grafted cultured freshwater pearls are all
formed from alternating circumferential layers of aragonitic nacre and organic
conchin, each will give the same response when examined with the endoscope.
Step 5: Detection of Value Enhancement Done
on Pearls
Another method in the
identification of a pearl is to establish whether or not it has been value-enhanced by techniques that are
presently considered to be either acceptable or unacceptable to the trade,
trade regulatory bodies and of course the buying public. Here the gemologist
must make the often too difficult distinction between what are considered to be
acceptable and unacceptable trade practices with respect to the value
enhancement of pearls. It is generally agreed that accepted trade practices for
the value enhancement of pearls include:
1) Traditional
‘peeling’ of natural pearls.
2) Tumbling
of newly harvested South Sea pearls in proprietary formulations designed to
remove residual residues and give the pearl’s nacre a marketable luster.
3) The routine bleaching and dyeing of akoyas before they are strung.
4) Whitening
pearls by bleaching their nacre.
5) Enhancing the luster of pearls by waxing their external surfaces.
However, many questions still
remain unanswered, with respect to how should the gemologist deal with many
other value enhancements for pearls that are being applied with increasing
frequency today?
In today’s pearl market, how
acceptable is the use of:
6) Dyeing
technology, to yield commercially acceptable black, golden or other
attractive colors in South Sea and mantle grafted freshwater cultured pearls?
7) Gamma
irradiation, and associated dyeing technology to darken and make much more
iridescent the colour of akoya, South Sea, and mantle grafted freshwater
cultured pearls?
8) Heat
treatment to induce golden hues in pearls?
9) Mechanical buffing and polishing to
improve both the shape and luster of the pearl’s nacre?
10) Various surface
coatings to enhance the luster of pearls?
The following guidelines for identifying value-enhanced pearls are
therefore offered for consideration and use.
A. Bleached and Dyed Pearls
Colour enhancement (by
bleaching followed by dye impregnation) cannot be detected readily on the
external surface of undrilled whole pearls. If a pearl is undrilled, laboratory
analysis by, for example, IR spectroscopy or Raman spectroscopy, will be
required to definitively identify the presence of any color-enhancing dye.
In contrast, color
enhancement, by dyeing, is detected relatively easily – once bead nucleated
cultured pearls, or tissue grafted freshwater cultured pearls have been
drilled. Examination of stringing channels or pegging channels in drilled,
bleached and dyed pearls will generally reveal:
dye
concentrated in the organic conchin layer between the shell bead and the outer
nacreous layer of dyed bead nucleated cultured akoya and South Sea cultured
pearls; dye staining the walls of drill channels in all bleached and dyed
pearls; and dye stained thread in some strung dyed pearls.
With the exception of drilled
silver nitrate treated black bead nucleated akoya cultured
pearls, which have an identifying X-radiographic appearance due to the
radiopacity of particles of silver impregnating the conchin layer surrounding their
bead; and dyed rough surfaced tissue grafted freshwater Biwa or Chinese
freshwater cultured pearls, which often have dye adherent to their external
surface, strings discolored by dye, and dye adherent to the walls of the drill
channels in the pearls.
B. Irradiated Pearls
The nacre of irradiated bead
nucleated cultured pearls, such as akoyas, tends to display greyish rather than black hues. If the drilled stringing channels
in these greyish irradiated akoyas are examined, they will be shown to have a
greyish to black shell bead that is covered by a comparatively thin layer of
white nacre. These colour-enhanced pearls also are inert to LWUV irradiation.
Other pearls, the colors of which are commercially value-enhanced by or other
forms of irradiation such as electron irradiation include off-colored bead
nucleated white South Sea pearls, and lower colour grade mantle grafted
freshwater pearls. Dark greyish South Sea bead nucleated pearls that owe their
colour to irradiation are not common. However, they do display the same identifying
features as irradiated akoyas.
Electron-irradiated Chinese
tissue grafted cultured pearls range in color from greyish to iridescent black,
depending on the radiation dose. These pearls are colored throughout, and are
inert to LWUV irradiation. Remember, however, black is not a natural color that
is found in Chinese mantle nucleated freshwater cultured pearls. Recently, the
pearl market has been inundated with black mantle grafted freshwater cultured
pearls, of up to 10 mm diameter, that have nacre that is highly iridescent.
These pearls are now known to
have been color enhanced by a still secret
Chinese process that allegedly involves a combination of dyeing with an
unspecified silver salt followed by irradiation. These treated black pearls
have the following identifying features: they are of unusually iridescent
nacre; they change color from black to dark
brown when examined in incandescent light; they show a loss of definition in
the ‘thumb print’ pattern on their nacre; and their nacre is inert to LWUV
irradiation.
C. Bleached Pearls
Both natural and cultured
pearls are bleached (with hydrogen
peroxide, or perhaps sodium
hypochlorite) to lighten overly dark hues and decrease the visibility of
brownish patches of conchin in their nacre. With the exception of the routine
bleaching followed by dyeing of akoya bead nucleated cultured pearls, how prevalent
is the use of bleaching among pearl producers is unknown. However, some
industry insiders suggest that up to 90%
of all white pearls have been bleached prior to their sale.
As guidelines with respect to
detecting bleached pearls have not been published, gemologists should carefully
examine the external surfaces of suspect pearls, at magnifications of 10–40, to
reveal tell-tale visual evidence that the nacre of the pearls has been
bleached. Increased surface roughness, due to chemical dissolution of margins
of aragonitic platelets exposed on the surface of nacre, and dissolution of the
organic matrix into which individual aragonite platelets have been deposited, will
identify the bleaching of nacre.
Examination of drilled
stringing channels in bleached bead nucleated pearls also will reveal that the conchin
layer between bead and nacre has been decolorized, and in some circumstances
either partly or completely dissolved by the bleaching agent. An additional
observation worth making is to examine the white nacre under LWUV. Bleached
nacre fluoresces uniformly a strong blue-white colour, while natural colored
nacre fluoresces in range bluish to greenish hues of quite variable intensity.
D. Treated Golden Pearls
Golden pearls created by
subjecting undrilled pearls to a
secret possible heating process
first appeared in markets in 1993. These treated golden pearls are claimed to
be color stable and that they can be polished without loss of their induced
color.
Gemologist specializing in
pearls has suggested that the possibly heat treated golden pearls can be
discriminated from pearls with natural colored yellow to golden-colored nacre
by the absence of absorption between 330
and 385 nm in their VIS reflectance absorption spectrum, the presence of concentrations
of color in surface defects on these pearls, and an uneven surface fluoresce to
LWUV not associated with the distribution of colour on the surface of the
pearls.
E. Waxed Pearls
Lower-quality pearls often
have their luster enhanced by the simple process of tumbling in a mild abrasive
to remove adherent external deposits, followed by the waxing of their external
surfaces to enhance the luster of the nacre. Fortunately, the presence of this
greasy coating on waxed pearls can be detected readily by 10 hand lens
examination, or by the mark left behind
when a fingernail is drawn across the surface of a waxed pearl.
F. Polymer-Coated Pearls
Both white and black South Sea
cultured pearls have the luster enhanced by deliberately coating the pearls
with thin films of colorless polymer. It is understood that this treatment is
performed in Japan, with thicker layers being applied to pearls of poorer luster.
Fortunately, polymer-coated
pearls can be detected with the naked eye by closely examining the luster of
the pearl, for polymer-coated pearls display
a rather muted orient beneath the polymer film, and on examination the gemologist
will find visual evidence of a thin coating of polymer and perhaps some
residual polymer attached to areas on the surface of the pearl. Evidence of
wear and scratching of the coating could also possibly be observed. It also has
been reported that polymer-coated white pearls glow pinkish on the top and
bluish on the side; while polymer coated black pearls glow purplish on the top
and green on the side.
G. Surface Polishing of Pearls
‘Pearl
skinning’ has been used traditionally
to improve the value of natural pearls by removing unattractive layers of nacre
from the pearl by hand and then trimming and polishing the surface. Today, it
is common practice to tumble-polish cultured pearls in mild abrasives to remove
adherent deposits, and enhance the luster of the pearl’s nacre by the mild polishing
action of that abrasive. However, when the presence of obvious scratches and
grooves on the external surface of pearls reveals that they have been ground
and polished to improve their shape and/or luster, a decision must be made
whether or not it is appropriate that surface polished pearls should be
identified on any gemological report.
H. Faceted Pearls
Faceted pearls are now an
acceptable fashion accessory, with some individual pearls being covered with
more than 150 triangular facets. This treatment is easy to detect and this
value enhancement is presently being applied to both white and black South Sea
pearls as well as mantle grafted freshwater cultured pearls of various hue.
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