The many varieties of pearls

Posted on June 09, 2012 by Brad Bilsten | 1 Comment

A pearl is a hard object produced within the soft tissue (specifically the mantle) of a living shelled mollusk. It is thought that natural pearls form under a set of accidental conditions when a microscopic intruder or parasite enters a bivalve mollusk, and settles inside the shell. The mollusk, being irritated by the intruder, forms a pearl sac of external mantle tissue cells and secretes the calcium carbonate and conchiolin to cover the irritant. This secretion process is repeated many times, thus producing a pearl.  The commonly held belief that a grain of sand acts as the irritant is in fact rarely the case. Typical stimuli include organic material, parasites, or even damage that displaces mantle tissue to another part of the mollusk's body and these small particles or organisms gain entry when the shell valves are open for feeding or respiration.

The ideal pearl is perfectly round and smooth, but many other shapes of pearls (baroque pearls) occur. A "natural pearl" or "wild pearl" is one that forms without any human intervention at all, in the wild, and is very rare.  Many hundreds of pearl oysters or mussels have to be gathered and opened in order to find even one wild pearl, and for many centuries that was the only way pearls were obtained and the reason that pearls fetched such extraordinary prices in the past.  Natural pearls are nearly 100% calcium carbonate and conchiolin. Natural pearls come in many shapes, with perfectly round ones being comparatively rare.

Cultured or farmed pearls from pearl oysters and freshwater mussels make up the majority of those that are currently sold and these parls are formed in a pearl farm, using human intervention as well as natural processes.  In cultured pearls, the irritant placed in the oyster is typically a piece of the mantle epithelium, with or without a spherical bead, thereby known as beaded or non-beaded cultured pearls.  Trade names for cultured pearls are Akoya, South Sea, and Black Tahitian

  • Akoya pearls are primarily grown in the cool to temperate saltwater of Japan (China is the second largest producer) in the Pinctada fucataoyster species. This temperature allows the pearl to develop highly uniform mineral crystals resulting in brilliant luster. Thus, many experts believe Akoya pearls have the highest luster of all types. It takes between 8 and 24 months for a pearl to grow, although most farmers wait at least one year in hopes of a larger pearl. The size can range from 2 to 10 millimeter (average size is 6-7mm) and the colors are rose, silver/white, cream, gold and grey/blue.
  • South Sea pearls are grown in the warm, pure waters off of Australia, Indonesia, the Phillippines, Mynamar and Thailand in two oyster varieties known as the Silver Lipped and Gold Lipped Oyster.  These pearls, and their shells, are the largest and rarest grown.  It takes 20 to 24 months to grow the pearl and there are many complications that can cause them to die.  South Sea pearls range from 9-20 millimeters (average size 13mm) and their colors include silver, silver/pink, white, white/pink, white/gold and gold.
  • Tahitian pearls are grown in the Pinctada margaritifera cumingi oyster species (also known as the Black Lipped Oyster) found throughout its native waters of French Polynesia. Only 1 in 10,000 of these oysters produces a pearl and because of this rarity, they cannot be mass produced. It takes 22 to 26 months for a pearl to grown and the size range from 8 – 18 millimeters (average size is 9-10mm), but there are some extremes. The largest Tahitian ever found was 25 millimeters! Tahitian pearl color includes peacock (the most popular), black/black, black/grey, silver/grey, black/rose, black/blue, black/green and aubergine (eggplant).

Salt water pearls can grow in several species of marine pearl oysters and live in oceans and are usually cultivated in protected lagoons or volcanic atolls.

Freshwater pearls are from freshwater pearl mussels that live in lakes, rivers, ponds and other bodies of fresh water.  They can be found not only in hot climates, but also in colder climates in countries such as Scotland (where they are protected under law).  However, most freshwater cultured pearls sold today come from China. Up to 40 pearls can be grown at once in one mussel and for a long time there was much emphasis on producing large quantities, but today science has improved the farming techniques resulting in higher quality freshwater pearls.  It takes 3 to 7 years for the pearls to grow and their size ranges from 3-13 millimeters (average size being 6mm).  Freshwater pearls come in a variety of pastel shades including white, pink, peach, lavender, grey, yellow and cream.  

Keshi Pearls are the dominant type of baroque pearls in the market today. The word "Keshi" is used in the Japanese language to describe something very small, for example a poppy seed could be described as "Keshi".  The three ways Keshi pearls develop are as follows:

1. Individual epithelium cells form the tiny pieces of mantle tissue (which has been implanted together with the nucleus) separate and move to another place in the soft body of the animal, where they multiply and form a pearl sac. This is where the pearl sack may proceed in the formation of a pearl.
2. Small broken pieces of shell drift into the animal during the nucleus implantation and injure the mantle tissue. The injury leads to the displacement of epithelium cells into the mantle's connective tissue, where they form a pearl sac.
3. The operated mollusk ejects the nucleus while the piece of mantle remains in its place and produces a pearl sac.

The outer shape of the pearls is baroque; this depends completely on the shape of the pearl sac, due to the lack of presence of a round nucleus. Keshi pearls often come in a half moon shape and their color often resembles that of Akoya cultured pearls. Some smaller Keshi pearls often have a white to silver overtone.

A popular aesthetic look for keshi pearls is to string together up to 50 strands of small pearls to form a substantial necklace. This look has been associated with fairly high class, and has stood the test of time enduring since the nineteen eighties. The price of keshi pearls is within the same realm as that of seed pearls. The harvesting and processing of these pearls is labor intensive.

Nucleated Freshwater Pearls that once existed in rumors were out in quantities for the world to see. They were not the round perfect spheres that pearl would expected but baroque pearls with tails. Unusual creatures but some were quite exceptional. It is very likely that bead nucleated freshwater pearls will be commercially produced in three years . The cultivation is in the trial stage at many farms with higher mortality rates and a high proportion of pearls with tails. If history is any indication, bead nucleated freshwater pearls will add a new chapter to the pearl world.

Imitation pearls are also widely sold in inexpensive jewelry, but the quality of their iridescence is usually very poor, and generally speaking, artificial pearls are easily distinguished from genuine pearls.  Some imitation pearls are simply made of mother-of-pearl, coral or conch shell, while others are made from glass and are coated with a solution containing fish scales called essence d'Orient. Although imitation pearls look the part, they do not have the same weight or smoothness as real pearls, and their luster will also dim greatly.Akoya Pearls
 

Gemological identification

The diagram below shows a comparison of a cross-section of a cultured pearl in the upper diagram and a natural pearl in the lower diagram.

   

A well equipped gem testing laboratory is able to distinguish natural pearls from cultured pearls by using a gemological  x-ray in order to examine the center of a pearl. With an x-ray it is possible to see the growth rings of the pearl, where the layers of calcium carbonate are separated by thin layers of conchiolin. The differentiation of natural pearls from non-beaded cultured pearls can be very difficult without the use of this x-ray technique.

Natural and cultured pearls can be distinguished from imitation pearls using a microscope. Another method of testing for imitations is to rub two pearls against each other. Imitation pearls are completely smooth, but natural and cultured pearls are composed of nacre platelets, making both feel slightly gritty. 

 

 

 

 

 

Information courtesy of www.imperialpearls.com and www.wikipedia.com

 

Posted in Akoya, baroque, Black Tahitian, cultured, Keshi, natural, pearls, South Sea


Treasure trove of emeralds donated to N.C. Museum in Raleigh

A sketch of a design by Brad Bilsten

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