How to Tell Real Pearls from Imitation Pearls

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.

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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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