The selection of black pearls

Black pearls can generally be divided into the following three types:

1. Natural black pearls

Nucleation cultured black pearls (cultured in Pinctada margaritifera cumingi, possibly some kind of mother-of-pearl). This oyster lives in the atolls of French Polynesia.

2. Rare brown to black seedless cultured pearls

The color variety of modified pearls is growing, including dyed black pearls, radiated pearls, and the latest so-called "laser treated" pearls.

3． Imitation pearls made from black shell.

Some rare imitations are made of the core surface covered with a layer of black essence de orient film, or made of black shells that are made into beads and polished to have a pearlescent luster. become.

Distinguish between natural black pearls and cultured black pearls, color-modified black pearls and imitations. Carefully observing the luster of coated pearls, we found that the pearlescent luster comes from underneath the polymer thin layer, while the pearlescent luster of untreated pearls comes from the outermost layer of the pearl. You can also observe the abnormal reflection of color on the top side of the pearl, which will also help in the detection of this kind of pearl. The color of this pearl has a pinkish tint from the top and a bluish tint to the sides; the black pearl treated this way has a slightly purple top and a greenish tint to the sides.

The characteristics of high-quality pure black pearls include:

1. The surface of the pearl is particularly smooth

2. The pearl feels sticky to the touch

3 The edge of the nacre superimposed platelet is difficult to see, and a layer of sharp tools and a colorless covering layer causing concavity can be seen.

This can be said to be the law of the market: as long as a certain product has good sales, there will inevitably be counterfeits of it. Black pearls are no exception. It is not difficult for us to see so-called black pearls in jewelers, but Some merchants forget to add the word dye before black. When you ask if these black pearls are real, profiteers can brazenly answer: These are real cultured pearls. Yes, they are made of real seawater or freshwater cultured pearls. It is dyed, but he should tell you that the beads have gone through a processing and dyeing process, just like a jade seller should tell you whether the beads have been optimized for B or C goods. Of course, it is not illegal in China not to tell you. But in black The main producing area of ??pearls---Tahiti (also known as Tahiti), if a businessman sells dyed pearls, his reward is to go to jail!

There are several common methods for pearl dyeing:

Silver salt treatment - soak pearls in nitrate and dilute ammonia solution, and then Reduction in strong light or hydrogen sulfide gas turns the color of pearls into black.

Ray irradiation——Place pearls under gamma rays of cobalt source to turn black.

< p>Use dyed bead cores - dye the bead core black before implanting it into the pearl oyster. When the pearl is formed, the black bead core makes the translucent nacre on the surface look black.

< p>Thin film coating - apply a thin layer of black glue on the surface of pearls.

It is not easy to distinguish dyed pearls, but it is not difficult to say. Master the following common sense , depending on the observation ability of your eyes, you should know that they are almost the same.

Color----Black pearls have different colors from light gray to black, and its beauty lies in the tone of its look. The accompanying color. Turn the black pearl slowly, and you can see the slight rainbow-like flash changing continuously. No two pearls are exactly the same. If you see a string (even a few) of jet-black color that is consistent and even, the iridescence will Rich black pearls, you are almost certain that they are dyed beads. The color of the dyed beads is dull, and the black color is darker where holes and flaws are gathered. This is easy to observe under a 10x magnifying glass.

Shape - Are the black pearls you see particularly round? Perfectly round black pearls are extremely rare and highly valuable. Typical black pearls are oblate, pear-shaped, oval, irregular, and many have Flaws or rings, so if the value-for-money black pearl you see is smooth and round, it is most likely dyed.

Size----Chinese or Japanese The diameter of seawater beads usually does not exceed nine millimeters, while the minimum diameter of black pearls is eight to nine millimeters, so round black pearls less than eight millimeters are mostly dyed beads.

Price----You Are the black pearls you see surprisingly cheap? An eight millimeter round black pearl costs more than a thousand yuan, while a dyed pearl of the same size is only worth one or two hundred yuan. If you are lucky enough to buy a black pearl at a reasonable price , Jinji requires the merchant to indicate on the receipt that the color of the pearl is natural.

If you want to buy expensive black pearls and are still not confident with the above identification methods, you might as well consider taking them to a jewelry appraisal unit. Use instruments to make a definite identification:

Ultraviolet fluorescence test----Under ultraviolet light, black pearls often fluoresce, with colors ranging from bright red to coffee red, but dyed pearls are more sensitive to ultraviolet light There is no reaction, or there is dull green fluorescence.

X-ray test - the film produced by X-ray can show the nacre of dyed pearls treated with silver salt. A circle of light white appears between the bead core and the bead core, proving its dyeing composition.

X-ray fluorescence test - pearls emit radiation of a specific wavelength under X-ray irradiation, and a spectrometer is used to measure the wavelength. degree, which can detect the chemical composition and the presence of silver salts.

Infrared photography test - use infrared film to photograph pearls. The image of black pearls is light blue, but it is processed with silver salts The image of dyed pearls appears yellow, or green-blue to yellow-green in color. Key points for identification of black pearls To identify a pearl or a pearl chain, conventional gem testing methods are rarely used. Instead, appraisers must combine observation of appearance characteristics with special testing methods, such as X-ray diffraction phase diagrams, reactions under UV irradiation, and advanced spectroscopy, to accurately determine where the pearl belongs. One type. To identify a single or a strand of black pearls, follow these steps.

Step one: Check the overall appearance of the bead chain. When the bead chain is being inspected, you should first hold the bead chain high with both hands and place it against a neutral background (light gray or milky white) for overall inspection. The white light illuminating above the necklace comes from a double-tube fluorescent desk lamp. Note:

1． Since natural pearls of the same shape and size are extremely rare, natural pearl strings are mostly graded by size, and different beads usually have different shapes. So, if a chain is made up of uniformly sized spherical beads, it is most likely a nucleated cultured pearl chain.

2. Comparing the color uniformity, the color uniformity of cultured pearl necklaces is much better than that of natural pearl necklaces.

3． Circles and crow’s feet are characteristic of cultured pearls.

Step 2: Detailed observation and judgment. First, the pearl and its internal drillings must be thoroughly cleaned. Use a soft cloth dipped in warm soapy water to remove sticky organic residue and oil stains. Once it is washed and dried, it will be easier to identify. Under good fiber optic lighting, you can use a 10x magnifying glass or low-power microscope for inspection:

1. Note the following about nacre: A ring-like toothed appearance can occur because when the outer surface of a natural or cultured pearl is broken, the thin layer covering the surface of the pearl is actually mother-of-pearl of aragonite lamellae. This appearance, which can be seen under a hand-held magnifying glass, is greatly different from the structure of coated essencedeorient imitation pearls. Its structure has a smooth, low-water-absorbing appearance. And this structure is a discontinuous and loose structure, covering the surface of an inner layer such as glass, plastic or shell beads. ...There is a thin layer of wax floating on the mother-of-pearl or it is difficult to observe the mother-finger ring-like appearance on the surface, then this kind of pearl is a laser-processed black pearl or a coated black pearl. …Beads with smooth surfaces and often polished marks are polished shell beads, which are a kind of imitation pearl. …Pearls with dye accumulation in surface pits are dyed seedless cultured pearls. ...Beads with obvious joint surfaces are natural semi-beads and cultured semi-beads.

2. It is best to use small-diameter fiber optic lights with adjustable brightness to illuminate the drilled holes of pearls. Pay attention to observation: ... those with thin onion-like concentric structures are natural pearls. ...There is a gray to white center shell bead, and there is a thin layer of brown to black organic conchyolin layer between the shell bead and the black nacre layer of the outer sphere, then this is a cultured black pearl. Black pearl drilling requires special care as it can determine which type of pearl it is: naturally colored black pearls or brown to black seedless (freshwater) cultured pearls. Nucleated black cultured pearls, black-dyed nucleated cultured pearls, black-dyed freshwater (nucleated and freshwater nucleated) cultured pearls, irradiated nucleated cultured pearls, and so-called laser-treated black pearls. For example, when using a magnifying glass of a certain magnification and adjustable fiber optic light illumination to detect black pearl drilling:

*Natural black pearls have an onion layer structure with thin black layers superimposed. *Nucleus-free cultured black pearls have a similar structure to natural black pearls, but may have a central cavity of uncertain size and shape. *Nucleated cultured pearls have a white to gray central bead core, which is first surrounded by a thin layer of black conchyolin, and then covered by a layer of black mother-of-pearl of varying thickness.

*Dyeed nucleated cultured black pearls have a black nacre and a black nacre. *Nucleated pearls that are irradiated to black are very characteristic, that is, they have a dark central pearl core and a slightly lighter, white to gray nacre.

*Lasered seedless pearls have a drilled hole that is stained black and is covered with waxy residue. …are imitation pearls that have no nacre and exhibit a usually thin artificial essencedeorient layer covering the surface of glass, plastic, or shell beads or hollow glass spheres. With a large central cavity and a pitted nacre around the string hole, this is a freshwater active nucleated (nucleated) cultured pearl, usually considered a Biwa bead or Chinese bead. Having two or more growth discontinuities (possibly different colors), it was recently revealed to be a Chinese freshwater cultured pearl in which the entire nacre has been dyed or laser-treated. 3. Observe pearls with transmission illumination. Place the pearl in the center of an adjustable-size aperture and let strong white light pass through the pearl. Using this method, you may see a striped (layered) structure, which is a core made of shells. structure. It is located at the center of an undrilled saltwater nucleated cultured pearl, and a similar layered structure can be seen in shell beads, a pearl imitation. Step Three: Direct Radiography Analysis Direct radiography can provide answers to questions such as: Are black pearls natural, cultured, or imitations? If the pearl is cultured, what type is it? Direct radiographic analysis has two major uses in pearl identification. Direct radiography can provide an approximate measurement (overall impression) of pearls that have become part of a necklace, especially those that have been strung on a string. Because of the rope, observation of the borehole was not possible. Second, in order to pursue the accuracy of identification of each pearl, the necklace must be broken off the rope, and photographed under X-rays one by one, and the structure shown in each picture must be analyzed and identified. Likewise, separate X-rays can be used to identify any unpunched pearl that is prong-set or bezel-set in jewelry. When pearls are examined by X-ray photography, the resulting picture is a light-dark negative (the white part shows the calcareous layer, and the black part shows the softer part). Negatives can leave a permanent record for identifying structural characteristics:

*Natural pearls... onion-shaped opaque layers (thin layers of aragonite) and transparent layers (arc or circular conchyolin layers alternated) Alternating structure, sometimes with a small transparent center.

*Nucleated cultured pearls...the central core area has poor ray penetration, and there is a thin transparent conchyolin layer wrapped around the core. The outer layer can be relatively thin (Akoya pearls) or thick (South Sea pearls) The nacreous layer has poor light transmittance. * Non-round active nucleated cultured pearls (Biwa or Chinese), non-nucleated cultured pearls... often have a weird shape that is impermeable to light, and sometimes contain a large, elongated transparent central cavity.

*Imitation pearls...are opaque or fully transparent spherical shapes and cannot distinguish light and dark structures.

When summarizing this step of pearl identification, it must be emphasized:

1. Direct X-ray photography is not of much significance for the identification of some pearls. For example: black South Sea pearls or natural pearls with a thick nacre layer often cannot see the arc structure or the transparent conchyolin layer concentrated in the center during imaging.

2. Analyzing X-ray images requires deeper theory and more practical skills.

3． Excessive X-ray imaging will reveal some structural details in the center of the pearl, while insufficient imaging will reveal structural details on the surface of the pearl.

4． The slight difference in photos of a less dense nacre and a denser core is often the only indication of a cultured pearl with a core.

Step 4: Optimize the detection of processed pearls. The fourth step in identifying a black pearl is to determine which method has been used to improve it. Whether it has been dyed, irradiated, laser Treated, coated, and even surface-polished shell beads. Dyeing and irradiation treatment In addition to nucleated cultured pearls dyed with silver nitrate (identification evidence can be found in X-ray photography: there are opaque point-like silver infiltrates around the Conchyolin layer on the outer layer of the pearl core), and dyeing With the exception of Biwa or Chinese active-core cultured pearls (which often have dye clinging to surface pits and imperfections), the color-improved outer layer of the undrilled pearl is not easily detectable. If the pearl is not drilled, the laboratory will be asked to identify any dyes used to improve the color through infrared spectroscopy and Raman spectroscopy. Relatively speaking, if nucleated cultured pearls or living nucleated freshwater pearls are drilled, it is relatively easy to identify the color improvement. For drilled and dyed pearls, if you check the holes of the string, you can find: *The dye stuck in the holes is nucleated cultured pearls *The nacre layer, if the conchyolin layer and bead core are dyed, it is black-dyed nucleated cultured pearls Pearl, this feature has obvious contrasting characteristics with nucleated black pearls. The nucleated black pearl has a white bead core and a black nacre layer, which can be seen from the drill hole. The mother oyster of the nucleated black pearl is P. margaritifera or the black-lipped pearl oyster. *The color of nucleated cultured pearls with black core and white nacre is obtained by gamma ray irradiation. *The dye of living nucleated pearls will stick to the holes or drilled holes *Dye attached can be seen on the strings between the pearls. The reaction of the natural black nacre to long-wave UV fluorescence is considered useful as it has not been treated. basis for instructions. For example, the nacre of natural pearls and untreated cultured pearls is naturally black, and under ultraviolet fluorescence, there is often a fluorescence of varying intensity but always dark reddish brown.

In contrast, dyed, irradiated and laser-treated black nacre is inert to long-wave UV light. Not much is known about this technology for laser-processed black pearls, as this type of pearl has only appeared on the Australian market in the past 2 to 3 years. The use of this technology can at least give Chinese freshwater seedless cultured pearls a beautiful black hue. It is doubtful that color improvement is achieved exclusively by laser irradiation. Because the heat generated by the laser will obviously carbonize the organic matter of the pearl, and it can also dehydrate the nacre and become dry and charred. I think that if laser technology can be used to improve this kind of iridescent black pearl, then to a certain extent, the energy of the laser can also be used to improve the dyeing process of adding moderate colors and the depth of dye penetration. When detecting laser-treated pearls, some unusual phenomena can be seen under magnification. Possible identification features include: l There is a strong halo effect on the surface of laser-treated pearls. The growth layers produced by the pearl's multi-stage growth process are perfectly improved. Pearls are covered with a waxy layer that is easily scraped off and often masks the concentric fingerprint-like pattern of the nacre. The brown waxy layer fills the flaws and spiral patterns on the pearl's surface. A layer of wax adheres to the pearl's drilled holes, masking the growth layer structure of the pearl's onion-like concentric rings. This treated nacre is inert under long-wave UV light.

We are currently incorporating Raman spectroscopy into our research, trying to discover the identification characteristics of this laser-treated pearl. Wax-injected pearls soak bamboo chips with beeswax for casting. They are often used to get cultured pearls just collected from fresh water out of the shells and remove the active organic matter stuck on them. This kind of wax-infused bamboo chips will Temporarily improves the luster of pearls. The wax attached to the pearl is easily detected under a handheld magnifying glass. Polymer-coated pearls. Some Tahitian black pearls (cultured) are covered with a colorless polymer (plastic) film in order to improve the luster of the pearl. We know that this process is completed in Japan and is made with A thicker layer of polymer covers the less lustrous pearl. This treatment method can be identified with the naked eye: ... carefully observing the luster of this coated pearl, it is found that its pearlescent light comes from underneath the thin layer of polymer, while the pearlescent light of untreated pearls comes from the outermost layer of the pearl.

You can also observe the abnormal reflection of color on the top side of the pearl, which will also help in the detection of this kind of pearl. The color of this pearl has a pinkish tint from the top and a bluish tint to the sides; the black pearl treated this way has a slightly purple top and a greenish tint to the sides. …examine the surface of the pearl to look for air bubbles trapped in the colorless plastic layer? As well as uneven surface morphology, evidence of clothing, scratches on the cladding, and evidence of dents that could be made on the surface with a tax tool. Polishing Pearls We know that sometimes we encounter some black South Sea pearls that have been ground with soft abrasives. Such as wood chips, beeswax-soaked bamboo chips, coke salt, and polishing materials commonly used to remove biomass from the surface of pearls. This polishing material can also be used to polish rough and dull areas on the nacre.

Although the above treatments are acceptable, people do not accept the process of polishing poor quality pearls with abrasive discs. Because this process will leave deep scratches on the surface of the pearl, this is also the basis for the identification of this kind of pearl. The question is: How can such common pearls be traded as having traditional nacre? In addition to natural pearls, will people accept this kind of pearls? Silica-Coated Pearls The characteristic luster of black nucleated cultured pearls has been improved by an external coating with polydisiloxane. Characteristics that should be noted when testing this improved pearl include: ...the surface of the pearl is particularly smooth...the pearl feels sticky to the touch...the edges of the nacre layer superimposed on the platelets are difficult to see, and a layer of sharp and concave colorless particles can be seen Covering layer.