Known in the gem trade as the “Queen of Gems,” pearls are possibly the first gems to be worn by human beings.  Perfect in form, pearls need not be cut and faceted like other gemstones to exhibit their singular beauty. 
Natural pearls form when an irritant, such as a boring worm, invades the shell of a mollusk—usually saltwater oysters or freshwater mussels.  Cultured pearls form after the irritant has been placed, by hand, in the oyster to promote formation of a pearl.  The oyster then performs its natural duty of forming nacre around the offensive irritant.  Nacre is composed of layers of overlapping crystals of aragonite mixed with conchiolin, a type of cement produced by the mollusk and that which gives pearls their distinctive iridescent appearance. The only way to confirm whether a pearl is natural or cultured is to expose the pearl in question to X-rays.     
Saltwater pearls have been harvested since great antiquity in the Persian Gulf and the Gulf of Manaar, which separates Sri Lanka (Ceylon) from the Indian sub-continent.  Until the 20th century, the only true pearls available were natural saltwater or freshwater pearls from various international oceans and streams; however, by the early 20th century, cultured saltwater pearls were being developed and subsequently mass-marketed. The industrial cultivation of freshwater pearls didn’t begin until after the end of World War II, when, using tissue from one oyster, it was found that if implanted in the correct position in another oyster, a pearl could form. 
Culturing of pearls began in China in as early as the first century B.C.E.  Production of “blister” pearls, where an object is placed on the inside of the shell and a half “pearl” develops over it, began in earnest in the 12th Century A.D.  At around the turn of the 20th Century, Japanese businessman Kokichi Mikimoto developed the first successful technique for culturing pearls, in which a bead-like shell nucleus is inserted into an oyster and nacre forms around the nucleus to produce a pearl.  This basic process is still used today in the production of saltwater nucleated pearls.
Over time, with the advent of cultured pearls, the natural pearl market began to weaken.  By the 1950s, the leading pearl product was cultured pearls, and this remains true today.  Culturing of pearls provides the grower with greater options in terms of the size, shape and color of the product they are offering.
Following is a listing of the most popular types of cultured pearls commonly available in the current market:
Saltwater Akoya Cultured Pearls
Primarily from Japan, the term “Akoya” refers to the specific mollusk used in the culturing process.  These mollusks are implanted with a nucleus or mother-of-pearl bead, and generally produce a white or cream-colored pearl with rosé or greenish overtones—what most people imagine when they hear the term “pearl.”  Although Akoya pearls can develop in almost any color, with the beautiful natural gray pearl being a rarity, they are generally of a muted hue, and the better qualities possess that typical pink or greenish overtone.  Often, in the best quality pearls, a rainbow effect is created on the pearl by the formation of an iridescent layer of orient.  Sizes tend to be from 1.0 - 7.0 mm. in diameter, with anything over 7.0 mm. considered rare. 
Freshwater Non-nucleated Cultured Pearls
Freshwater non-nucleated cultured pearls are produced in large quantities in China, although initially they were grown in freshwater lakes in Japan.  Freshwater pearls are rarely completely round, but are produced in a myriad of shapes, sizes and colors, most frequently in shades of white, cream, gray, pink, peach and lavender.  Also produced in limited quantity in the United States, these freshwater pearls possess their own particular charm.  
South Seas Cultured Pearls
South Seas cultured pearls, which derive from Australia, are extremely large pearls grown from a larger species of oyster.  South Seas pearls first appeared with any regularity on the market in 1960s.  These larger pearls typically possess a very white body color with silvery overtones, and can grow from 7.0 to 20.0 mm. in diameter, generally being found at 12-14 mm. in diameter.  They rarely occur in cream and gray colors.  Golden South Seas cultured pearls, primarily produced in the Philippines, are a variant of the same oyster as the South Seas pearls, called the Gold-lip Pinctada Maxima.  All of these larger pearls can occur either in round, drop, circle or baroque shapes. 
Tahitian Black South Sea Cultured Pearls
Tahitian black South Sea cultured pearls are another variant of oyster, producing gorgeous black and dark gray cultured pearls throughout the Tuamotu Archipelago region of French Polynesia.  When first introduced to the market in the early 1970s, these large black pearls were hard to sell since people thought the color couldn’t be natural.  Once the color was proved to be of a natural origin, the sales of these black beauties skyrocketed.  The black and dark gray body colors of these pearls provide the perfect background for green, bronze and plum overtones, and exceptional orient.  Tahitian black pearls are produced in sizes from 8-20 mm. in diameter, with the most popular sizes ranging from 8-12 mm.  Any pearls over 15 mm. are rarely available.   
Keshi pearls
Keshi pearls are a result of the pearl culturing process.  If an oyster rejects the implanted nucleus or mantle tissue, a void or pearl sac is left behind where the nucleus or tissue would be.  Keshi pearls develop in these “blank” pearl sacs and because they are solid nacre without a nucleus, generally possess superb luster and often orient.  Keshi pearls can occur in salt or freshwater oysters and tend to be irregular in shape, since there is no nucleus to guide or form a specific shape.  Although Keshi pearls were once readily available, they are now becoming a rarified breed, as the pearl growers now x-ray their oysters to see if the implant has taken.  If not, they re-implant the oyster, leaving no room or time for a Keshi pearl to grow.
Soufflé pearls
A new type of pearl appeared at the 2010 Tucson Gem Show called Soufflé pearls.  These are Chinese freshwater cultured pearls that are grown after the first harvest of pearls has already taken place.  The oyster is left with a pearl sac that is re-implanted, this time with a small amount of pond mud rather than the usual nucleus or mantle tissue.  The pearl uses the mud as a nucleus and grows around it.  The result is a baroque pearl with magnificent luster and orient.  Metal oxides present in the mud believed to be the cause of the creation of the extraordinary luster and orient present in these pearls.  When Soufflé Pearls are harvested, a hole is drilled to remove the excess mud that might still be present inside the pearl.  Soufflé Pearls are also much lighter than most of their cultured counterparts due to their hollow centers. 
Mabe pearls
Mabe pearls are a type of cultured blister pearl.  Blister pearls form on the inside surface of the shell. When harvested, the blister pearl is cut from the shell and then filled with plastic and closed in the back with a fitted piece of mother-of-pearl to provide a finished look.  Mabe pearls are usually in the size range of 10-18 mm. in diameter.
Pearl Grading
Grading the quality of pearls consists of examining the size, shape, surface, color, luster, orient, nacre thickness, and, if appropriate, matching
Size - Generally the size range of natural pearls and cultured Japanese Akoya pearls is from 1 to 10 mm. in diameter.  The larger the pearl, the less spherical it tends to be.  Round saltwater cultured pearls over 7 mm. are considered large, and sizes from 15 to 20 mm. are rare unless considering South Seas and Tahitian cultured pearls.  A general rule is that with all other quality factors being equal, the larger the pearl the more valuable it is.
Shape - Regarding spherical pearls, the rounder the better.  The Japanese phrase for an “eight-way roller” pearl refers to a pearl that can roll in any direction due to its exceptional roundness.  Pear or drop shape pearls are particularly popular for use in earrings and pendants.  The more equilateral the pear shape, the higher the value.  In the last thirty years, however, baroque (non-spherical) shapes have become quite popular and have risen steadily in exposure and value.
Surface - The ideal pearl possesses no surface blemishes or dimples and appears completely smooth.  The smoother the surface, the greater the luster of the pearl.  Most pearls do not have an absolutely smooth surface, however, and those containing blemishes are drilled at the flaw for use in strand necklaces, rings, etc.  Completely smooth pearls are rare and very highly prized.
Color Pearls come in white, off-white, cream, golden, peach, pink, black, silver, lavender, and blue.  Color preference in pearls can be subjective and is usually matched to the complexion of the person wearing the pearls.  Pearls have the unique ability among gems to give a subtle glow to the skin.  White pearls with rosé overtones are generally preferred by those with light complexions, whereas darker complexions are enhanced by wearing golden or silver pearls.  Black or near-black pearls look good next to any complexion. 
Luster – This refers both to the quantity and quality of light reflecting off the surface of a pearl.  Luster is manifested as the reflection of the light source on the center surface of the pearl.  The sharper and more intense the reflected light source, the greater the luster quality and the more valuable the pearl.
Orient This is the phenomena on the surface of pearls that exhibits iridescence.  Orient can be predominantly one color such as green or pink, or consist of a rainbow of colors in weak, moderate, or strong appearances.
Nacre Thickness – This term refers to the iridescent substance (aragonite mixed with conchiolin) produced in layers by a pearl mollusk that surround foreign irritants that  enter the shell.  Nacre thickness more than any other property listed here is responsible for the durability of an individual pearl.  Generally the thicker the nacre layer, either around the nucleus of a cultured pearl or around a hollow or solid area in the middle of a natural pearl, the greater the luster, durability and value of the pearl.
Matching - Pairs of pearls that are identical in size, color, shape and quality command a higher price than two single unmatched pearls of equal quality.  Matching groups and whole strands of pearls of equal color, shape and quality can require anywhere from a few months, for the more commercial strands, to many years, for a fine gem-quality strand of matched pearls.  Because pearls are produced from a living organism, it is harder to control the quality of the final product.
When handling pearls it is important not to get the silk thread they are strung on wet.  This can weaken the thread and cause the necklace to break.  Pearls should only be cleaned by wiping gently with a soft clean cloth.  It is advised not to spray perfumes or oils onto the pearls as this can cause the pearls to change color or peel.  Pearls should be re-strung at least once a year if worn often.
Pearl Properties
Species:  Pearl
Varieties:  Saltwater, Freshwater, South Seas, Tahitian.
Chemical Composition:  Calcium carbonate + conchiolin (C32H48N2O11) + water
Colors:  White, Off-White, Cream, Yellow, Golden, Green, Blue, Lavender, Pink, Peach, Silver, Black.
Hardness:  2 ½ - 4 ½
Crystal System:  Orthorhombic (microcrystalline)
Refractive Index:  1.52 - 1.66, Black - 1.53 - 1.69
Specific Gravity:  2.60 - 2.85
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