- Types and Varieties of Cultured Pearls
- Oyster Culture
- Grading Cultured Pearls
- Pearl Growth
- Sources of Cultured Pearls
- Physical & Optical Properties of Cultured Pearl
- Identification of Natural and cultured Pearls
- Valuation of Cultured Pearls
Probably the most popular and widely used substitute for any gem is the cultured pearl. A multitude of problems have occurred in the natural pearl industry to reduce their availability to a level far below potential demand. It has been fortunate that cultured pearls of fine quality have been available to fill the void.
Although the production of cultured blister pearls had been accomplished successfully for centuries, the spherical cultured pearl was unknown before the 20th century. Early in this century three Japanese experimented intensively in an effort to produce spherical pearls. Although his important position in the industry led many to believe that the late K. Mikimoto developed the spherical cultured pearl, research on the subject by Dr. A.R. Cahn, of the U.S. Bureau of Fisheries, prior to the publication of his note worthy report on the pearl culturing industry ("Pearl Culture in Japan", Fish & Wildlife Service, U.S. Department of Interior, Washington, D.C.), showed this impression to be incorrect. It seems that two other men, Tatsuhei Mise and Tokichi Nishikawa, were responsible for the spherical cultured pearl and arrived at the same correct conclusions at almost the same time independently. Mise, a carpenter, had his interest in pearls awakened by his stepfather after the latter's return from a government sponsored trip to inspect oyster beds off Australia. Mise started pearl culturing experiments, although he had no training to prepare him for such an endeavor. Cahn stated in his report: "Available information points to the conclusion that he was the first person to develop a spherical cultured pearl". His first successful effort, a tiny pearl formed about a lead nucleus, was apparently completed prior to 1904. He applied for a patent early in 1907, but his application was refused. In October of the same year, Nishikawa, a trained zoologist who left a position in the Japanese Bureau of Fisheries to experiment on spherical pearl culture, applied for a patent on a very similar method it was granted nine years later and seven years after his death. Although Nishikawa's application was applied for later than Mise's, Mise's was ruled as an infringement of Nishikawa‘s. In 1908, however, Nishikawa signed an agreement with Mise that made the use of the two methods common property, thereby seemingly acknowledging the prior success of Mise's endeavors. From the nature of the Patents obtained by Nishikawa, it seem that his method was the more practical of the two. " Nishikawa was not on friendly terms with Mikimoto, although Mikimoto was his father-in-law. After Nishikawa's, however, Mikimoto made a working arrangement with Nishikawa's son, and since that time the Mikimoto firm has used the general method of spherical-pearl culture.
This short account of the background of pearl culture in Japan fails to consider the great contributions to the development of the industry that were made over the years by Mikimoto. Although he cannot be credited with the nearly simultaneous discoveries of Mise and Nishikawa, he is the one who may be created with crating the cultured-pearl industry as it exists today. The methods of rearing young oysters and most of the farming methods explained later in the assignment are his.
The terms CULTURED and CULTIVATED are both used for whole, artificially propagated pearls. An examination of the dictionaries discloses synonymous or closely similar meanings for both words. In Japan, pearl farms are cultivated in the sense of tending them in a manner similar to that in which farmers tend their agricultural crops. To many persons, however, cultivated flowers mean more desirable flowers than wild ones. Any term that will confuse the public as to the real character of a substance is to be avoided. The use of either term can be construed either as an aid or a detriment to the desirability of these pearls, depending on one's, understanding of the meaning of the word. Among its, more usual definitions, the term "culture" implies an improvement or a refinements a better product. In the biological definition of the world, however, is found the firmest ground for its preference, since this definition indicates organic growth, stimulated and controlled by man. Dealers in natural pearls prefer the term cultured to cultivated, believing that it is less misleading. The French were the first to have litigation resulting in terminology: "perle de culture" for the Japanese frauds and "perle fine" for natural pearls. This was unfortunately translated into English to mean cultured pearl. The English-speaking French man never says cultured pearl; he says CULTURE PEARL, a product of pearl-oyster culture. Some members of the pearl trade regret that they did not adopt the term "plated pearl" or "veneered mother-of-pearl" to indicate the similarity of cultured pearls to plated gold.
Because K. Mikimoto was long believed to have been the originator of cultured pearl, the habit of calling them "Mikimoto pearls" became widespread. The use of this term by retailers may result in either discriminatory publicity for one organization or in the sale of cultured pearls not produced by that organization as being their product. Therefore, the use of the name Mikimoto as a descriptive term for cultured pearls should be avoided.
The better qualities of cultured pearls are very desirable and beautiful. The observable portion possesses the luster, texture and other external characteristics of a true pearl. As with synthetic stones, their introduction marked a marvelous improvement over the best of the former imitations.
Although the public is already unusually interested in pearls because of the educational efforts of the producers of the cultured product, some of these efforts have resulted in implications that have given the public incorrect ideas. Gemological students are in an excellent position to clarify these misunderstandings and to establish and maintain a lasting interest in both cultured and natural pearls.
Cultured Whole Pearls
These are produced by the insertion of a mother-of-pearl bead into the mantle of the pearl-bearing mollusc, Pinctada Martensii. The nacreous coating usually has a thickness of about one-half millimeter; often less, occasionally more.
Cultured Blister or Half Pearls.
As explained in the natural pearl page, a blister pearl is a natural calcareous formation that grows attached to the shell of the mollusc. A cultured blister pearl is one whose growth is artificially induced. As early as the 13th century the Chinese used mud pellets or spherical mother-of-pearl beads to stimulate the formation of blister pearls. They also inserted knob like pieces of bone, wood or brass or small leaden images of Buddha. The shell was gently opened with a spatula of bamboo or pearl shell, and these foreign bodies were placed in rows on the inner surface of both lips of the shell by means of a forked bamboo stick. The molluscs were tended in shallow ponds adjoining the river, and after a few years the growths were cut out.
Today, blister pearls are produced by inserting a half-bead against the shell. A bead of much larger diameter than could normally be tolerated within the mollusc can thus be used. The resultant formation is cut from the shell, the half-bead removed, and the nacreous half-dome is cemented over a mother-of-pearl bead of appropriate size and shape. The result is called a "mabe" or "Mabe pearl".
Many whole cultured pearls have unsightly blemishes. Removal of these disfiguring marks by sawing produces half pearls or three-quarter pearls, depending on the amount of materials removed.
Cultured Pearls Without Nuclei - Biwa Pearls
Fresh-water pearl culture is being conducted in Hirako Reservoir, an arm of take Biwa, in Shiga Prefecture, Japan. A fresh-water clam of good size is used as the host mollusc. This species requires seven years to reach operable size. It was found early that large nuclei caused a 50% death rate and that less than 10% of the pearls produced were saleable. As a result, small squares of mantle tissue are now inserted in place of a nucleus, about thirty being used for each clam. The pearls that are obtained three years later are quite irregular in shape, averaging six by three millimeters, but have fine color and luster. These fresh-water non-nucleated cultured pearls have become important in the cultured-pearl market.
Recently, some success has been achieved in northern Australian waters by inserting mantle tissue in salt-water molluscs, but the result has not reached marketable quantities. As with those produced in fresh-water clams, the resultant pearls are nearly always baroques.
Although wild molluscs are still collected annually for use in cultured-pearl production, the main supply comes from molluscs raised from the cultured SPAT on the pearl farms (Note: a spat is the free swimming larval stage of development that ends when the young oyster attaches itself permanently to a solid surface.) Wild three - and four year old oysters are collected by diving girls operating from small launches and skiffs.
At the gathering period several hundred to a thousand people are so employed. The oysters are gathered from beds, one to ten meters in depth (one meter = 3.28 feet) and taken to pearl-culture farms, where they are distributed over the bottom in shallow water in areas from which all other oysters have been removed. This is done in early Autumn, and the oysters are undisturbed until the following Spring. They are then collected, but only the healthy ones receive the nucleus necessary for cultured-pearl production.
About 1924 the collection and rearing of very young oysters started and has since become the most important source of molluscs. Experiments by Mikimoto demonstrated that the freely moving spat, just before settling to the bottom to attach itself to some undersea object, develops a sensitivity to light that makes it seek dark areas. Therefore, he developed a darkened cage. The cages have dimensions of about 33" x 20" X 8", formed by covering a heavy wire frame with small wire mesh. Each cage has several shelves of wire mesh. The wires in the cage are given a rough surface by dipping them in tar and a mixture of cement and sand. Black boards are then fastened to the sides and bottom of the cage to produce the necessary darkened area. The cages are suspended at a depth of about six meters below the surface. The spawning period is from July to September, and the cages remain out until late in November. Fantastic numbers of spherical pearls collect in each cage, the average being about 7,000 to 10,000. The young spats are transferred from the collecting cages to rearing cages in November. The rearing cages protect the young oysters effectively from their natural enemies, so a high percentage survive. At the age of about one year the young oysters, which have grown by this time to about one inch in shell diameter are sown in water over a faintly rough bottom. Nothing further is done to them for about two years. During the summer months of the third year, the oysters are collected by women divers and brought to barges where the shells are selected for nucleus insertion and cleaned of incrusted organisms. Undersized shells are returned for another year of growth, and those that are too distorted or too old are discarded.
After the oysters have reached the age of three years, they are collected. Those in which nuclei are to be inserted are cleaned and brought to women technicians for the performance of this delicate task. Usually, the oysters are transported to the area in baskets that hang in the sea from rafts located near the scene of the operation. The molluscs are induced to spread the two sides, or "valves," of their shells by a variety of methods. The major producers generally place them on the wharf near the technicians, where many will open in a short time. Another method is to place them in trays of stagnant or running water. A bamboo wedge is then inserted to keep the shell in the open position. The operation must take place shortly after the wedge is inserted; more than two hours of enforced opening is likely to result in fatigue and probable death after the operation. The wedging operation must be done with great care, because breaking the shell edges or other careless handling may cause a dangerous reduction in the animal's vitality and a resulting decrease in the quality of the pearls, even if the mollusc survives.
At tone time it was though that it was necessary to sew the bead into a sac of mantle tissue before placing it in the host mollusc, but this has been found unnecessary. Today, the tissue containing the essential epithelium cells is still obtained from the mantle of living molluscs that have been selected for this purpose, but only that portion of the outer surface of the mantle facing the shell is used. It is cut into small squares, each of which is large enough to cover about one-third of the nucleus. To produce the finest quality pearls, it is imperative that these squares of mantle tissue be in contact with the nucleus and so oriented that the original outer surface (i.e., that one on the shell side) is in .contact with the nucleus. The sac that is formed around the pearl is produced by the regeneration of the epithelial cells of the outer edge of this piece of mantle, the same cells that construct the shell of the mollusc.
The nucleus is prepared from large fresh-water mother-of-pearl shell found in the United States along the Mississippi and some of its tributaries this has proved most satisfactory from the standpoint of thickness and strength. Small cubes are cut from the shell and abraded into spheres by rotating them between sheets of iron; they are then polished by further abrasion in cotton bags. The ultimate size of the cultured pearl is determined almost entirely by the size of the nucleus.
As the first step in the operation, the opened mollusc is clamped to the technician's table. She folds back the mantle tissue and makes an incision in the foot of the animal and opens a small channel into the main mass. The graft tissue that was prepared earlier is then passed down the incision followed by the nucleus, which is placed just above the mantle tissue. The foot mass is then smoothed back to close the opening and the wedge is removed, unless a second operation is planned. Another nucleus can be inserted elsewhere in the body, but much greater care and skill is required for a second operation. It is also possible to insert a third nucleus, but extreme delicacy is required and it is rarely attempted, except with small sizes.
The molluscs that have been operated on are placed in special cages suspended from rafts anchored in sheltered water. After four to six weeks the oysters are inspected and those that have not survived the ordeal are removed. The cages are then moved to a permanent position and suspended from rafts at a depth of two to three meters.
Here they remain undisturbed for three to four years, except for occasional removal of encrusted growths from the shells. This cleaning operation is done at least three times a year. After each cleaning, they are placed in freshly tarred cages. Harvesting is started in October and continues until after the first of the year. Some operators who feel that nacre deposition in the cold-water months is more lustrous and adds materially to pearl quality delay their harvesting until just before Christmas.
The molluscs are protected as much as possible from natural enemies by the cages, but certain enemies are difficult to guard against. Sudden temperature chances effected by the vagaries of ocean currents may be damaging. Such animal enemies as the starfish, ell and octopus feed on the pearl oysters, if given a chance, but the cages afford excellent protection against them. Barnacles and seaweed cause damage that can be vented only by cleaning the cages. Occasionally, the so-called red tide caused by tremendous numbers of plankton (a minute marine organism), bring about wholesale destruction of pearl bearing molluscs.
Any animal life that uses gills for breathing is likely to be dealt a fatal blow by a red tide. Fortunately, however, this phenomenon is comparatively rare.
After the pearls have been removed and cleaned carefully to avoid scratching, they are graded for size; this is done with sieves. Next they are counted and graded for quality. The first grading sorts the pearls into good, medium and poor categories, indicated by the letters A, B and C. Then a more exact grading separates them into Aa, Ab, Ac etc. The A classification usually includes all spherical or nearly spherical pearls that have good luster and are not discolored. B includes baroques of good color and luster and spherical or nearly spherical shapes of medium color and luster. Those in the C grade are of little or no value for gem purposes; they are used primarily for the pearl "medicine" that is valued so highly by the Orientals. Usually, only the three A grades are utilized for better quality necklaces; B grades are used mostly for rings, brooches, buckles and other ornamental jewelry.
Pearls to be used for necklaces are first examined for blemishes and marked with black ink to indicate the starting point of the drill hole. Drilling is done with a 3/4 millimeter steel wire ground to a triangular point. The drill is mounted in a chuck attached to the spindle of a small electric motor. The pearl is held in a chuck that can be moved forward as the hole is made. A hole is drilled slightly beyond the center and then completed from the opposite side. Centering to ensure that the two holes meet properly is accomplished by inserting a small bamboo sliver in the first hole and centering to a small hole in the back of the pearl chuck. Drilled pearls are matched for color, luster and size and selected in pairs from large to small. Each pair is placed side by side in grooves on black baize-covered trays. The large center pearl is then selected. Stringing is done on silk thread with a very thin needle. Final grading and evaluation is done by an expert appraiser and the weight and quality is recorded. Strands are tied together in groups of one hundred and reweighed for marketing in lots of that number. The standard unit of weight for cultured pearls is known as a MOMME (pronounced MOMM-ey), which is equal to seventy-five pearl grains or 18.75 metric carats.
Reference is often made to "seven-year", "nine-year" or "twelve-year" cultured pearls. The average life span of the average Japanese mollusc is eight years, and the best nacre production occurs between the ages of three and seven years. As explained previously, nuclei are not introduced until the mollusc has reached the age of three and attained sufficient size. Thus, for the average Japanese mollusc, the maximum time for nacre accretion is five years. However, since the nacre produced during the fifth year may detract from the beauty of the pearl, the usual maximum is four years. In actual practice, since 90% of nacre accretion takes place in the six months when water temperatures are highest, pearls are left in the mollusc from the beginning of the growing season to the end of the season, three and one-half years later. However, some may be left in five or even six years, if the mollusc survives. If the shape, color and luster are good, such pearls are more valuable than the usual three-and-one-half-year pearls. Therefore, references to seven or more years of nacre accretion to explain size or quality differences are incorrect.
The size of a cultured pearl is determined not by the time it is in the mollusc but by the size of the nucleus. The rate of nacre accretion in Japanese pearls is near .15 millimeter per year, whether the nucleus is small or large. The largest Japanese cultured pearls reported had diameters of approximately eleven millimeters and nuclei of about nine millimeters. In equatorial waters, however, where higher water temperatures exist the year around, nacre accumulation is much faster (up to ten times the Japanese rate), and pearls of significantly greater diameter have been produced. In addition, the larger host mollusc used means that the size limitations are raised considerably. Cultured pearls in the 11 to 15 millimeter and larger size range are exclusively South Seas products. At the other end of the size range, some as small as one millimeter in diameter have been produced; however, the minimum commercial size is about one and one-half millimeter.
Layers of nacreous and prismatic crystalline materials, the same as those that comprise natural pearls (with perhaps fewer conchiolin layers), form the artificially propagated layers that cover the nucleus. The structure of a mother-of-pearl nucleus of a cultured pearl is similar to that of the concentric layers that form a true pearl, in that both substances are composed largely of tiny prismatic crystals. However, mother-of-pearl differs from the true pearl substance, since it is formed in nearly plane (flat) layers of prismatic substance.
To intimate that all or even any appreciable number of cultured pearls contain only a small nucleus of mother-of-pearl similar in size to that of genuine pearls is obviously untrue and unethical. Cultured Pearls do occasionally have comparatively thick nacreous coatings, but they are usually strung with those having layers of average thickness.
Many salesmen of whole cultured pearls have intimated that the producers have successfully used seed pearls instead of mother-of-pearl for the nucleus. However, proof has never been submitted to the institute that pearls of commercially important size have been produced by the use of a seed-pearl irritant.
Japan is the major source of cultured whole pearls. The culture farms are located in Mie Prefecture (Ago Bay, Gokasho Bay, Matoya Bay, Nie Bay and others), as well as Nagasaki, Wakayama and other prefectures as indicated on the accompanying map. Minor quantities of cultured blister pearl are produced in Japan and China.
Experimental culture stations were operated by Mikimoto before World War II at the Palau and Celebes Islands in the South Pacific. Palau was chosen because of the varied colors of natural pearls found in that area, and the Celebes station was established because of its production of very large natural Pearls. Because it was difficult to keep the mollusc alive long enough to produce attractive pearls, both stations were commercial failures. However, Japanese firms have established several facilities in recent years. Among those mentioned have been Australia (in Brecknock Harbor, between Augustus Island and the northern coast) and a Burmese location. Because of the many problems attendant to production in equatorial areas, however, the quantity of cultured pearls available from these new sources is very limited.
|Chemical Composition||The outer nacreous layers have the same composition as the natural; Calcium carbonate, CaCO3.|
|Crystallographic Character||Same as the natural.|
|Hardness||21/2 to 4|
|Toughness||Same as the natural, If the outer nacreous layers are too thin, they may crack or wear through quickly to the artificial center.|
|Specific Gravity||2.72 to 2.78; heavier than most natural pearls but lighter than most of fresh water origin. The high S.G. is caused by the density of the mother-of-pearl nucleus the S.G. of which is 2.78 or more. An exception is non-nucleated cultured pearls, which can be even lighter than the natural.|
|Degree of Transparency||Same as the natural|
|Luster||Same as the natural|
|Refractive Index||Same as the natural|
|Birefringence||Same as the natural|
|Optic Character||Same as the natural|
|Phenomena||Same as the natural|
|X-Ray, Fluorescence||Weak but distinct yellow|
|Absorption Spectra||Varies Widely|
Effects Caused by:
|Heat||Same as the natural|
|Acids||Same as the natural|
Of all the gem materials, pearls pose the greatest difficulty from the standpoint of testing. Because of the numerous problems involved, there are many times when the jeweler's only sensible alternative is to submit the pearls to a properly equipped laboratory for identification.
The most common and most dangerous error is to attempt to use some of the indications, which are unreliable at best for making an identification. Even the large jewelry store is in a considerably different position with respect to testing pearls than it is for testing colored stones and diamonds. The X-ray equipment that is necessary to identify pearls beyond any doubt is useful almost exclusively for that purpose; moreover, it is expensive and dangerous to operate, except by fully trained technicians. As a result, almost all of the pearls tested in this country are tested by gemological laboratories. The only two with units modified to identify pearls by the conclusive X-radiographic method are the Gem Trade Laboratories of the Gemological Institute of America in Los Angeles and New York City.
There are few jewelers today who handle natural pearls in quantity; in fact, few handle or sell them at all. A significant proportion of the cultured pearls sold are inexpensive varieties that show enough typical blemishes and other characteristics to permit a trained man to satisfy himself that a necklace is cultured. In other worlds, although individual pearls would be difficult to identify, an average of characteristics in a strand may be sufficient to give a strong indication of their origin. Thus, a poor quality cultured-pearl necklace is usually readily detectable by simple methods, whereas a fine cultured strand can seldom be distinguished from a natural strand by the usual simple methods. Since the expensive cultured and natural strands are seldom sold by the average jeweler, however, an inability to distinguish between them rarely causes a problem; when difficulty is encountered, the strand can be sent to either the New York or Los Angeles laboratory of the institute.
There are so many "old-wives" tales connected with pearl testing that it is essential to distinguish between those that are reliable in every case, those that offer some indication of the identity of a strand, and those so-called tests that are utterly unreliable. First, we will examine a number of features that are usually visible on cultured pearls, particularly those in the lower price ranges. These features are usually seen in diffused white light without benefit of magnification, Japanese cultured pearls are said to be characterized by appearance differences that are detectable to one who handles pearls and cultured pearls in great quantity. These are:-
- a gelatinous appearance, caused by the transparency of the coating;
- black welts of irregular shape under the surface, caused by an irregular deposit of the dark-brown conchiolin before nacre deposition started;
- protrusions and depressions on the surface; and
- often a faint greenish cast.
Surface appearance, however, is a very doubtful method of attempting to distinguish cultured from natural pearls, for even the experienced may be deceived by good quality cultured strands or poor naturals. It is useless for single pearls.
There are other characteristics that can be used to gain an indication of the majority of pearls in a strand that are somewhat more reliable than those mentioned above; these are revealed by special lighting. The most effective method of examining low-quality cultured necklaces is called CANDUNG. This method requires an opaque light shield into which is cut a circular opening slightly smaller than the diameter of the pearls to be examined. The shield is placed over an intense light, and each pearl is rotated over the circular opening.
Most cultured pearls with relatively thin nacreous coatings will reveal a striped effect at some point as they are rotated this is caused by unequal transmission of light through the flat mother-of-pearl layers in the large nucleus. Cracks near the surface of a natural pearl may produce a similar appearance, but if the majority of pearls in a strand exhibit the striped effect, it is likely that they are cultured. A fine quality cultured strand may well have nacreous coatings that are too thick to reveal the striped effect. Failure to detect these stripes may mean anything; a cultured pearl with a coating of normal thickness, a natural pearl, a weak light source, etc.
If the coating is very thin, merely rotating the strand slowly beneath a strong light source may disclose a difference in the amount of light reflected from two sides of the mother-of-pearl nucleus. The best way to describe the resulting effect is to liken it to the adularescence of moonstone.
Although the detection of either the stripes or the moonstone like reflection leaves something to be desired from a scientific viewpoint, since neither can be considered a positive test, it is certainly reasonable to assume that the strand is cultured when the vast majority of pearls in the strand exhibit either effect. If all other evidence, such as subsurface black welts and a bumpy surface, points to cultured origin, a cultured pearl strand is likely. Absence of any such evidence, however, should never lead one to an inference that the strand is natural, since these indications suggest only the cheapest cultured strands.
Many simple tests for distinguishing cultured from natural pearls have been published from time to time. Those of little or no practical value are mentioned here to warn of their unreliability, even as indications of identity. For example, specific gravity is often suggested, because the average cultured pearl is very slightly more dense than the average salt-water natural pearl, however, they overlap almost completely. The mean S.G. for Persian Gulf pearls is about 2.71 (range, 2.66-2.76), and the average for nucleated cultured pearls is about 2.75-2.76 (range 2.72-2.78). Non-nucleated cultured pearls will overlap the densities of natural pearls. Australian natural pearls range from 2.66 to 2.78. The only way that this test could be used with any value would be to adjust a liquid to a 2.73-2.74 density to test all the pearls in a strand. If all floated, the chances would be in favor of a natural strand, if all sank, cultured. For individual pearls, the test is useless, and a mixed strand is an ever present possibility. The heavy liquids may cause the pearls to deteriorate at an appreciably increased rate.
Fluorescence under ultraviolet radiation of either the usual long wavelength or short wavelength type, either at the surface or in the drill hole, has no diagnostic value.
A comparatively large number of natural pearls have a surface appearance that is sometimes described as an "orange-peel" or "hammered" effect. However, certain cultured pearls, particularly those with every thick coatings, frequently exhibit a very similar surface appearance.
Another test that is mentioned frequently in books on gemstones is the "pearl compass", which depends on the reaction of minerals to a magnetic field. The pearl is suspended between the poles of a powerful electromagnet. All mineral substances (in some degree, however, slight) are either attracted by a magnet and tend to align their longest dimensions parallel to the magnetic field (paramagnetic), or are repelled and tend to set their longest dimensions at right angles to the class. Since it is round, but normally composed largely of a nucleus made up of flat layers, a cultured pearl in a strong magnetic field tends to rotate until the longest dimensions of the mother-of-pearl core are arranged at right angles to the line of magnetic force. A natural pearl because of its concentric nature throughout, remains stationary when tested under identical conditions. Although this is theoretically plausible, actual testing by this method has proved quite indefinite and of little practical value, due perhaps to the structural variations in both natural and cultured pearls.
Several methods have been used to improve an observer's field of view when examining the drill-hole walls of a pearl. The first attempts were made by using as a reflector the surface of column of mercury introduced into the drill hole. Most present observations are made with instruments incorporating a metal needle, the end of which has been cut off at a 45° angle and highly polished to act as a mirror. These instruments are variously known as ENDOSCOPES, PEARLOSCOPES or PEARLOMETERS, and the techniques are called ENDOSCOPIC METHODS. Some endoscopic methods provide only indications of identity, but one is completely reliable. First, those providing indications will be discussed. One method is to observe the drill hole while the pearl is illuminated from the side (Figure 2). This allows the wall of the hole to be examined under magnification while the light is being diffused by the body of the pearl. In a genuine pearl, varying layers, somewhat similar to the rings tin a tree, are generally seen to continue to the center of the pearl. In a cultured pearl the passage of the needle from the coating to the nucleus is marked by a sharp change in the intensity of the light reaching the mirror, caused by the relative opacity of the dark conchiolin layer. A natural pearl shows a gradual diminishing of light until the mirror reaches the center, after which it gradually increases in intensity. In a cultured pearl the tree-ring effect ends at the conchiolin layer, unless the drill hole is nearly perpendicular to the mother-of-pearl layers.
Another method of using the endoscope is to observe the pearl from the side while directing the light into the drill hole to reflect off the 45° mirror. This is actually a reversal of the procedure described above. It proves most satisfactory when little or no magnification is used. Figure 3 illustrates the effect of a mother-of-pearl nucleus on light reflected from the mirror needle: the light is trapped between two layers and much of it is carried to the surface of the pearl, where it appears as an indistinct chatoyant or adularescence effect. When a natural pearl is tested in this manner, the light follows its concentric rings. In its transmission the light is gradually scattered and absorbed; therefore, the pearl appears comparatively evenly lighted from the side (Figure-4).
If, instead of a single reflective surface, a hollow needle with a pair of mirrors inclined at 45° is employed, as in Figure 5, still further differences between natural and cultured pearls can be observed. When such a hollow needle is inserted into the drill hole of a cultured pearl and an intense beam of light is directed through the needle, the light strikes the first mirror and is reflected to the wall of the drill hole, where it is trapped between the layers of the mother-of-pearl core and carried to the surface of the pearl. This is illustrated in Figure 5. If a pearl is natural, the effect produced is entirely different. As the pair of mirrors reach a point where they are equidistant from the center of the pearl, light from the first mirror is carried by the concentric rings to the second mirror and a flash of light is noted on the surface of the second mirror (Figure 6). Unless the microscope is used to observe the reflections from the second mirror, this effect is almost impossible to see. The double-mirror method is absolutely diagnostic when the flash of light is seen; it is proof of natural origin. When no definite flash is seen, a pearl cannot be identified by this method, although the usual presumption is that it is cultured, since it is a rare natural pearl that fails to show this flash of light. The main disadvantage of this testing technique, however, is that the pearl must be drilled and each pearl in a strand must be tested individually. This is not economically feasible for routine testing because of the time required. There are several other devices designed for the examination of the drill hole of a pearl, but all are quite similar in construction and, function.
A brief discussion of pearl testing by X-rays is presented below, followed by a summary of the relative value of all tests that can be applied to pearls.
X-ray tests can be divided into three principal types: DIFFRACTION, LUMINESCENCE and RADOGRAPHY.
This involves the projection of a small pencil beam of X-rays through the pearl. The beam is diffracted by the planes of atoms within the calcite or aragonite crystals through which it passes and produces spots on an X-ray film placed behind the pearl. The spots ion the film are oriented in relation to the structure of the crystals. The manner in which the crystals are arranged in a pearl of ideal structure is shown in Figure 7A; the structure of a typical cultured pearl is Illustrated in Figure 7B. The long prismatic aragonite crystals that comprise much of the structure of a pearl diffract the beam so as to produce a hexagonal pattern on the film when the beam passes parallel to their length (Figure 8), and a rectangular pattern when passed at right angles to their length (Figure 9). In a natural pearl with a perfectly concentric structure, a beam directed through the center of the pearl always travels parallel to the length of the long prismatic crystals, since they are always oriented perpendicular to the pearl's surface and radiate from its center. In contrast, a typical cultured pearl contains a core of mother-of-pearl, which has parallel layers of these crystals; thus, there is only one direction in which a hexagonal pattern can be obtained. Any beam passing in a plane that is other than perpendicular to the layers and parallel to the length of the crystals, or nearly so, produces a rectangular pattern.
Since the cultured pearl may show the hexagonal pattern of spots in one direction, it is necessary to take a second shot, after a rotation of 90° on each pearl showing the hexagonal pattern.
Diffraction equipment may be designed to accommodate about a dozen pearls at a time, but each strand must be unstrung and the entire process is very time consuming. The equipment is prohibitively expensive, as well. South Seas cultured pearls with excessively thick nacreous coatings could give results indistinguishable from natural pearls. In addition, the method is valueless for non-nucleated cultured pearls. Theoretically, at least, structural irregularities could cause some natural pearls to give reactions expected from cultured pearls. Thus, although one or two European laboratories regard X-ray diffraction as the most conclusive means of pearl identification, the institute has distinct reservations regarding its value.
The vast majority of cultured pearls fluoresce vividly when subjected to high-voltage X-radiation, a phenomenon that is caused apparently by the excitation of manganese in the fresh water mother-of-pearl core. When used alone, however, certain exceptions reduce the effectiveness of this test:
- Almost all fresh-water pearls also fluoresce, since they are composed of essentially the same elements as the shell.
- A large percentage of natural pearls from the Australian fishing areas fluoresce, although very weakly.
- Very rarely, natural pearls from other areas display fluorescence under X-rays.
- Dyeing agents reduce the intensity of fluorescence. Certain cultured pearls that have been dyed with metallic compounds do not exhibit fluorescence, because the metallic components of the dye mask the fluorescence to such an extent that it is not observable to the X-ray technician.
- Very thick coatings of nacre mask the fluorescence of the core. This applies to the rare South Seas cultured pearl.
This method is useful in conjunction with radiography, which is explained in the next few paragraphs, for it provides a quick indication of the identity of a strand of pearls. If all of the pearls fluoresce, the strand is certain to be made up of fresh-water or cultured pearls; if none fluoresce, an Oriental necklace is suggested. If some do and others do not, a mixed strand is in prospect and the luminescent pearls may be marked to facilitate separation after confirmation by X-radiography.
Photographic MethodThis involves the transmission of a wide beam of X-rays through a pearl or necklace of pearls and onto a film to produce a RADIOGRAPH, or shadow image of its structure. Conchiolin, which comprises the main division of each of the layers of crystals, is more transparent to X-rays than the remainder of the pearl. As a result, the portion of the beam passing through the conchiolin suffers less absorption and creates a stronger image on the film. The resultant pattern made by the onion like layers of the pearl can be studied to show its true nature. This method of testing provides POSITIVE results, with the right equipment, clear radiographs, and interpretation by qualified personnel. A radiograph of a natural strand is shown in Figure 10, and one of a cultured strand in Figure 11. The interpretation of pearl radiographs requires considerable experience and a sound knowledge of the various structural peculiarities of both natural and cultured pearls.
Fluoroscope MethodIn this method a visual examination of the shadow image of the pearl is made on a fluorescent screen. It is faster because it eliminates the need of developing an exposed film. It is disadvantageous, however, because the images cannot be studied with enough care in the limited time of exposure; moreover, no permanent record can be obtained for future reference. In actual practice, only a very small percentage of cultured or natural pearls exhibit a sufficiently clear shadow on the screen to permit interpretation by the most capable observer. Therefore, this can be considered as a theoretical method that has not proved satisfactory in any respect. Many errors have been traced by the GIA to those who have been known to use this method.
The various testing methods used in the identification of pearls and cultured pearls can be summarized as follows:
- Careful X-radiography by the photographic method provides POSITIVE PROOF of the identify of unknown pearls. It also provides a permanent record for future reference.
- X-ray luminescence provides SUPPLEMENTARY information that is helpful, even though it lacks the reliability of the radiographic technique.
- X-ray diffraction provides positive proof in MOST cases but may give false results, as indicated earlier. Therefore, it may be considered as a SUPPLEMENTARY test.
- The Fluoroscope method employed with wide-beam X-rays is UNRELIABLE for discriminating between pearls and cultured pearls because of the lack of contrast and definition in the shadow image.
- The double-mirror endoscopic method provides reliable information as to the origin of NEARLY ALL DRILLED Oriental pearls, but only an INDICATION with cultured pearls. The remaining endoscopic methods are not reliable.
- All of the other tests described provide only PARTIAL INDICATIONS of identity.
The value of cultured pearls is affected by the same factors that control the value of Oriental pearls i.e. color, luster, orient, shape, size, etc., plus other factors that apply to the thickness of the nacreous coating. Those with thin outer coatings may crack under sharp blows or heavy pressure and are therefore not desirable. Also, this thin coating may wear away completely and expose the raw cores, especially at the end of a necklace and with persons who have acid conditions. Cultured pearls thus damaged cannot be repaired, whereas natural pearls might be. The thickness of the coatings can often be determined in the drill hole by the position of the discontinuation layer. As with natural pearls, the size of the drill hole should be only sufficiently large to accommodate the stringing thread.
An increasing number of cultured pearls are being artificially colored pink, blue, black and other colors to disguise the greenish tint that is frequently present and that some regard as undesirable. One method consists of dipping poor-quality pearls in a brownish-black dye; the dyeing solution, however, is confined to the surface and obliterates any luster and orient that may have been present. A more effective method involves the injection of a chemical in a half-drilled pearl. A reaction takes place over a period of time from the inside out, making the color penetrate throughout. This treatment, which is done primarily in France, is superior to dyeing, because luster and orient are unaffected; moreover, the color is more attractive and does not fade. An additional method for blackening cultured pearls involves the use of silver salts. The pearls are put into a silver-oxide powder for weeks, then into a silver-nitrate solution. The free silver thus deposited leaves a black color, plus some of the luster and orient.
The following list of cultured-pearl qualities may be used in conjunction with the accompanying price charts:
MatchingAlthough the term very finely matched implies exact matching, it actually refers to the blending of cultured pearls that have very similar qualities: all pearls in a given section of a necklace will appear to be the same in all of their qualities. Also, the necklace as a whole will present a similar overall appearance. Exact matching is usually done only on Paris pearls. If two cultured pearls are matched exactly, their value will be somewhat more than the sum total of the two pearls before matching.
LusterWhen a cultured peed is described as having a very fine luster, it means that it has a high luster of even intensity (i.e. equal in all areas of the pearl).
ShapeA cultured pearl described as round in shape would theoretically, be perfectly spherical. In practice, however, it refers to a pearl that is sufficiently round to appear spherical to the eye.
Nacreous CoatingA well coated cultured pearl produced in Japanese waters will have a nacre thickness of approximately 0.5mm, Nuclei left in the oyster one year will be covered with only 0.15 mm, of nacre or less.
Freedom From All FlawsAs the statement implies a gem quality cultured pearl should be free from flaws of any kind.
ColorThere are two colors that are the most desirable: (1) pink rose, having a pink body color with pink overtone; and (2) white rose, which is pure white with a pink overtone. (Note: This pure white must not be confused with the kind of white pearls that lack translucency and orient.) South Seas cultured pearls nearly always have a brownish, golden, white or silvery body color.
The price charts list the wholesale prices for cultured-pearl strands of both graduated and uniform sizes and the per pearl prices for loose, undrilled cultured pearls. The millimeter measurements across the top of each chart refer to the size of the pearls in the strand; in the case of graduated strands, they refer to the size of the smallest and largest pearls in the strand. For uniform strands, two sizes are given, reflecting the 1/2 millimeter gradation from the slightly larger center bead to the ends of the strand. The millimeter figures on the chart of undrilled pearls refers, of course, to the size of single pearls.
The figures in each square indicate the approximate price range for a given size and quality of strand. Not included, however, are prices commanded for exceptionally fine qualities. (Note: The word STRAND refers only to a number of pearls strung together; the word NECKLACE describes a strand of pearls PLUS CLASP.)
For the following reasons, the prices of loose, undrilled cultured pearls show a greater variation than the prices of strands: (1) Drilling can usually be planned so as to eliminate certain flaws, but an already drilled pearl does not have this advantage; (2) single pearls or small groups of pearls are usually more carefully examined and graded than strands; (3) smaller quantities are being sold.
Button, pear, egg and drop shapes, if perfectly formed, can be evaluated by using the weight as the determining factor. For example, an ideal button shape would have a value approximately equal to that of an ideal round pearl of the same weight, all other factors being equal. (Refer to table in entitled "Weight Estimations of Round Natural and Cultured Pearls").
Although cultured pearls are usually considered to be less expensive than natural pearls, some of exceptional quality have been sold or offered for relatively high prices. Not too long ago, a New York retailer sold an exceptional strand of cultured pear for $30,000.