Pearls are formed in molluskan bivalves (clams, oysters, mussels) of several species by the secretion of a substance known as nacre around an irritant in the outer tissue (mantle) of the organism, or between the outer tissue and the shell. Nacre is composed of an organic, horn-like compound called conchiolin, and calcium carbonate (CaCO3) in the form of the minerals aragonite or calcite. All species of bivalves are capable of producing pearls, but only a few can secrete a nacreous coating that has the attractive pearly luster of gem pearl. Nacre is also the substance that coats the inner surface of bivalve shells. In gem pearl-producing bivalves this nacreous shell lining also has a pearly luster and is called mother-of-pearl.
Nacre is secreted in a succession of concentric, very thin, translucent layers around the irritant. The aragonite within the nacre is present as slender crystals that are preferentially oriented perpendicular to the surface being deposited. This radial and onion-like arrangement of aragonite and nacre is highly regular and is the source of pearls iridescence and unique luster. The body color of a pearl, which can range from gray-black to creamy white (see below), is caused by the conchiolin present in the nacre. Conchiolin is most frequently brown, but its transparency and color is a function of the chemistry of the water in which the bivalve lives. Because conchiolin is translucent, the color of the aragonite (most often white or pale yellow) shows through, yielding a white or cream colored pearl. Impurities in the water in which the bivalve lives can give rise to yellowish, pinkish, reddish, or blackish-gray pearls.
The rate of deposition of nacre to form a pearl depends on the species of bivalve, but in most species is very slow (approximately 0.6 mm/yr.). Moreover, it has been estimated that natural pearls form in only about 1 in 40 pearl oysters, so that it is apparent that natural pearls of large size are indeed rare, particularly considering the limited number of bivalve species that are capable of producing gem pearl and their average life span (3-11 years for the Persian Gulf pearl oyster).
The shape of a pearl depends on two factors: 1) the shape of the irritant nucleus around which the nacre is deposited; 2) whether or not the pearl has moved during its growth. An irregularly shaped nucleus or shifting of the pearl during growth can both lead to an irregular, or baroque shape. Most natural pearls are not perfectly round for these reasons.
The most historically important source for fine natural pearls is the Persian Gulf, where pearl oysters were once found in great abundance off the coast and islands of present day Bahrain. The creamy white pearls from these oysters are still considered the finest natural pearls in the world and command premium prices. Other historic and present-day sources for fine natural pearls are the Gulf of Manaar (Sri Lanka), and the Red Sea. More recent important sources include the north and northwest coast of Australia, with lesser production from Burma, Tahiti, New Guinea, Borneo, and Venezuela. Pearls of different colors are often associated with their place of origin; Persian Gulf pearls are creamy white; Sri Lankan are a paler white; Australian are white or silvery; Tahitian and Mexican are gray-black to reddish-brown; West Indies are rose red with wavy lines; "Indian" (actually Sri Lankan) may be a faint rose color.
Cultured pearls are pearls that grow around a nucleus that has been manually placed within an oyster or mussel. The practice of inducing pearl growth is an ancient one, dating back to the 13th century Chinese, but was only perfected in the early part of this century by the Japanese. Prior to the advent of modern culturing techniques, a round nucleus, usually composed of clam or oyster shell, was placed between the the mantle and shell of the bivalve and allowed to grow for 2 or 3 years. The nacre which coated the pearl also coated the shell lining, resulting in a pearl that was attached to the shell (a blister pearl). In modern cultured pearls the nucleus, which is a bead composed of freshwater clam shell wrapped with a strip of the mollusks mantle, is placed within the mantle or gonad of the bivalve, away from the shell, allowing it to grow freely. Blister pearls, which are hemispherical in shape, are used today in ear rings, rings, and brooches, where only a half-pearl is necessary. A variant of cultured blister pearl is the Mabe Pearl, an assembled cultured pearl made by removing the nucleus of the blister pearl, painting the inside of the nacre coating with dye or pearl essence (see below), filling the opening with epoxy or other resin, and covering the base with a piece of mother-of-pearl.
The size of cultured pearls depend on the size of the nucleus used, the species of mollusk, the temperature and chemistry of the water, and the time allowed for growth. Several pearls may be grown in a single oyster, but because the amount of nacre secreted by an oyster is constant, the size attained by multiple pearls in an oyster is smaller than for a single nucleus over the same time. Japanese culturing techniques vary, but in general nuclei remain in the oysters from 2 to 7 years before being extracted. For "nucleated pearls", those containing a shell bead as a nucleus, only about 1 in 4 oysters produce a cultured pearl. For non-nucleated pearls, a strip of mantle is inserted into the mantle of a freshwater mussel. About 80-90% yield pearls using this technique.
Cultured, non-nucleated, freshwater pearls (generally referred to as simply freshwater pearls, despite the fact they are cultured) are small and irregularly shaped, resembling rice krispies in form. These are typically harvested after 12-18 months of growth; those from the Lake Biwa region of Japan are referred to as "Biwa" pearls.
Japan has traditional been and continues to be the largest producer of both nucleated and non-nucleated cultured pearls. The single most important more recent producer is Australia, where warmer coastal waters and a slightly larger indigenous species of oyster currently yield cultured pearls of exceptional color (silvery white), size (up to 10-12 mm in diameter) and luster. These are sometimes referred to as "South Sea Cultured Pearls" and are distinctive for their size and color. In recent years China has begun to produce commercially significant quantities of cultured freshwater pearls and should become a market force in the future.
Polynesian Cultured Black Pearls
Oysters capable of producing black pearls are indigenous to the waters off Peru, Baja California, Panama, certain islands of Indonesia, Micronesia, the Philippines, Okinawa, and French Polynesia (Tahiti and adjacent islands). Only from the latter have cultured black pearls been produced in large numbers, and this only in the last 30 years. French Polynesia had historically been an important source for natural black pearls until about the beginning of this century, when the oysters were all but eliminated by overharvesting. Using Japanese techniques, black pearl culturing began in 1966 and by 1972 about 1500 grams of black pearls had been exported. In 1988, 446,827 grams of black pearls, worth an estimated 22.8 million dollars, were exported. With the increased availability, the popularity of black pearls has soared but they remain relatively rare and thus expensive.
In contrast to the common white cultured (Akoya) pearl, which rarely exceeds 9 mm in size, Polynesian black pearl oysters, which are larger than their Japanese counterparts, typically produce 8-14 mm pearls from bead nuclei 5-9 mm in diameter. The growth of nacre is also more rapid (2-2.5 mm/2 years), yielding pearls in two or more years of superior luster.
Reliably and accurately distinguishing natural from cultured pearl is difficult, and is usually done with an x-ray radiograph. This can reveal whether the nucleus comprises the majority of the pearl (as in most cultured pearls) or how thick the nacre is. A simple but nondiagnostic test is S.G.; most cultured pearls will sink in a liquid of density 2.74, whereas most naturals will float. Several other, in some cases nondefinitive, features are often useful in distinguishing cultured pearls from naturals:
Cultured pearls can be (and are!) dyed black by soaking in a weak solution of silver nitrate and dilute ammonia, followed by exposure to light or H2S gas. This changes the color of conchiolin (permanent) and renders the pearls virtually indistinguishable from natural or cultured blacks. Nearly all(?) black pearls on the market prior to about 1978 were of this kind, or had been rendered black by irradiation (see below). Dyed black pearls can be distinguished from natural and cultured blacks by u.v. fluorescence; most untreated blacks glow an indistinct reddish brown in long u.v. light, whereas most dyed blacks do not fluoresce or show a weak, slightly greenish color under long u.v.. Dying can sometimes also be detected by the present of minute specks of concentrated color that are visible under 10X or greater magnification. This unevenness of color contrasts with the diffuse, even color distribution in naturals. Cultured blacks are also usually larger than the cultured white pearls that are dyed (see above) so this can serve as a tip-off as well.
Processes to produce black pearls from Japanese cultured white Akoya pearls by exposure to a cobalt gamma ray source were patented in 1960 and 1963. Unlike dying, exposure to cobalt radiation changes only the color of the freshwater bead nucleus and does not affect the color of the nacre. It is thus ineffective on pearls with a thick or nontransparent nacre, or those having other than a freshwater shell nucleus. Drilled irradiated blacks can be detected by examining the drill hole to see if the nacre is black or white. Dyed and irradiated blacks can be detected by u.v. fluorescence and examination of the bead through the drill hole, inasmuch as untreated cultured blacks have a white nucleus bead with a dark gray nacre and fluoresce a dullish red in long wavelength u.v.. Size is again a good gauge because most cultured blacks are 9 mm or larger, whereas blackened Akoya pearls rarely exceed 9 mm.
Dying cultured white pearls to pink is also a practice that has seen widespread use, but can be detected in drilled pearls by examining the drill hole to look for an uneven concentration of color. The dye is often concentrated along the bead-nacre interface and is not as dark in the nacre proper.
White cultured pearls are bleached to remove any natural yellowish cast and improve color uniformity. F.T.C. guidelines do not presently require disclosure of bleaching or pink dying to the purchaser.
Imitation pearls are constructed from a wide variety of beads (plastic, wax, glass, composite materials) that are coated with substances having pearl-like lusters. One such substance, called pearl essence or essence d'orient is made from fish scales. All imitations lack the rough texture of natural or cultured pearl and have markedly different specific gravities.
Pricing and Valuation
Natural pearl weights are given in carats and grains; a pearl grain is 0.25 carats. "Seed" pearls are those that weigh less than 0.25 grains. The weight unit for Japanese cultured pearls is the momme, which is equal to 3.75 grams (75 grains) and is equivalent to the weight of a 14" strand of 3mm cultured pearls.
Well-matched strands of natural pearls of nearly any color or size are prohibitively expensive for all but the most affluent. Cultured pearls, which are grown by the millions every year in a broad spectrum of qualities and sizes, sell for one-tenth or less of the price of naturals and have become the norm.
Except for large pearls and baroques, most pearls are sold at the wholesale level in strands that are collected into bunches. Bunches are given a base price which is multiplied by a grading factor that includes the number of pearls in a given size range and their weight. The total price for a bunch is given by the base price multiplied, for each size range, by the weight in grains squared, divided by the number of pearls in each size range.
By purchasing bunches of strands, dealers are able to avoid the higher import duties of ready-to-wear necklaces. The strands are graded and matched at their source of origin by experts who match and price strings according to size, color, roundness, luster (function of thickness of nacre), and cleanliness. Each of the factors influencing price is described below.
Though not universally standardized, cultured pearls are usually categorized into 4 or more grades. Common designations are AAA (Extra Fine), AA (Fine), A (Good), and BB (Commercial). AAA pearls show a very uniform color distribution, a strong orient (iridescence), a very high, uniform luster, are essentially free of flaws, and are highly spherical. AA differ by showing a less even color, luster and/or orient, a lower luster, a very slight spotting, preferably close to the drill hole if strung in a necklace. Matching in necklaces within both these categories should be very good. Tahitian black cultured pearls are similarly ranked.
"A" category pearls may be the same body color as above, but of poorer luster and lacking an orient or iridescence. Flaws may be visible to the naked eye on close inspection and shapes may be slightly out of round. Matching will not be as good in necklaces of this grade.
"BB" pearls may be somewhat chalky white with low luster, no orient, and lacking the translucent quality seen in the above categories. Color may be spotty, surfaces may show blisters, shapes can be noticeably out of round and in necklaces the color and sizes may not match as well.
The principle use of round or near-round pearls in jewelry is in necklaces. These are strung in two main varieties; graduated necklaces, where the size of the pearls increase from each end toward the center, and uniform, where all pearls are nearly the same size, with perhaps a few smaller pearls at the ends. Graduated necklaces are classified as having either compound or straight taper, depending on whether, when laid flat in a suitable groove, a ruler placed along one edge of the pearls from the small end to the center touches all pearls (straight taper) or just a few (compound taper). The type of taper is a matter of personal preference; either can be constructed as individual taste dictates.
Prices for necklaces vary depending on the goodness-of-match within the string and the quality and size of the pearls, as described above. Necklaces of well-matched pearls of uniform diameter are more expensive than graduated necklaces. Graduated necklaces are usually sold in 19" lengths, whereas uniform necklaces are generally available in several lengths, each with its own trade name (i.e. choker=15-16"; princess=18"; matinee=22-24"; opera=30-36"; rope= >36"). A necklace composed of multiple strands of small pearls is called a "bib".
Care and Treatment
Of all gems, pearls are the most susceptible to damage from abusive treatment. They are porous, slightly rough, and soft, which means they can be easily scratched or discolored by contact with nearly any liquid (soapy water included) and skin oil.