Pearl

In affiliation with GemologyOnline.com
From The Gemology Project
Pearl
Chemical composition Calcium carbonate, conchiolin and water
Crystal system Amorphous
Hardness 2.5-3.5
Refractive index 1.52-1.69
Specific gravity 2.68-2.86
Lustre Pearly
The Baroda Pearls
Photo courtesy of Christie's Images LTD. 2007


The most famous pearl necklace is the 2 strand Baroda pearl necklace (known as "The Baroda Pearls"). It is made of 68 graduated (9.47 to 16.04 mm) natural pearls, all matched to color, luster, size and shape. It is assembled from the original 7 stranded pearl necklace owned by the Indian Maharajas of Baroda.
The Baroda pearls were sold in auction at Christie's New York for USD 7,096,000 an all time auction record (April 25th, 2007).

Contents

Formation

Pearls are, usually, created by certain types of mollusks, which we shall refer to as oysters or pearl oysters, when an inorganic or organic material is embedded between the mantle and the inner shell of the oyster.

The mollusk group is a large group of (mostly marine) animals, such as oysters, clams, mussels and marine snails. Land snails, slugs and squids also belong to this group. In gemology mostly the marine univalve and bivalve animals that have decorative shells or are able to create commercial viable pearls are important. Cuttlebone from cuttlefish (also a mollusk) is used as a casting medium in jewelry.

The mantle is the outer organ of the oyster which covers most of the shell. The outer cells of this mantle (called the epithelium) are responsible for the secretion of nacre and conchiolin (AKA mother-of-pearl).

Mother of pearl with drillholes caused by living organisms
Photo courtesy of Parels-AEL

Natural pearl formation is very rare and usually happens when a parasite or other living organism drills its way through the shell of the oyster, where it will then encounter the mantle. The mantle is irritated by this parasite and tries to defend itself by encapsulating the irritant with epithelial cells, forming a sac of cells covering this parasite. These cells will then secret layers of nacre and conchiolin on the parasite.
Conchiolin is an organic substance made by the epithelial cells that act as a glue between different layers of nacre (a calcium carbonate).

Mostly this process takes place in the mantle of the oyster and free pearls of all shapes are formed. When an irritant is attached to the inner shell, then epithelial cells will deposit conchiolin and nacre on the irritant and it will form a blister on the inside of the oysters' shell. To no surprise the latter type of pearls are named blister pearls.

There are other irritants that may induce pearl formation, but the story that they originate from sand grains is a myth. Although it could be possible that a grain of sand gets trapped between the mantle and the shell, it is very unlikely. Oysters live in sandy environments and they simply "spit" any dirt out.

Formation of natural pearls


In the image on the left the formation of a natural pearl is illustrated.

1. An irritant is trapped between the mantle and the innershell of the oyster.
2. The epithelial cells start to divide and try to encapsule the irritant.
3. The irritant is now fully enclosed with epithelial cells and has moved inside the mantle. This creates a sac of epithelial cells around the irritant. Such a sac is named a pearl sac.

From the outside in, the pearl sac (the cells) secrete nacre and conchiolin unto the irritant and the pearl starts to grow. The pearl sac expands through cell division as the pearl grows.

In this illustration a "free" pearl is formed. When the irritant attaches itself to the shell, then there will be no formation of a pearl sac and the epithelium cells just deposite nacre and conchiolin on the irritant and the inner shell, creating a blister.

A pearl sac is essential in the formation or free pearls, for both natural as cultured pearls.


Types of oysters

Inside of a Pinctada margaritifera shell
Photo courtesy of Parels-AEL

There are many oysters (bivalve mollusks) that can form pearls, either living in fresh or saltwater. The species that produce pearls of commercial value are listed below.

  • Pinctada martensii (Akoya pearls)
  • Pinctada maxima (South Sea pearls)
  • Pinctada margaritifera (Black or Tahiti pearls)
  • Pteria sterna (Sea of Cortez pearls®)
  • Pinctada mazatlanica (Mexican pearls)
  • Pteria penguin (Blister pearls - Mabe)
  • Haliotidae haliotis (Abalone)
  • Hyriopsis cumingi (freshwater "Zebra mussel")
  • Cristaria plicata (freshwater "Cockscomb mussel")

There are also other mollusks (gastropods) that can produce pearls without nacre. Some people do not regard these as real pearls (due to the lack of nacre), but the formation is strongly related to that of oysters and the "pearls" they produce are very rare.

  • Strombus gigas (Conch pearls), a marine snail
  • Melo melo pearls, a marine snail
  • Mercenaria mercenaria (Quahog or "Hard clam")

When the pearl formation is natural without any intervenience by man, we name them natural or "oriental" pearls. Otherwise we refer to them as "cultured" pearls.

Cultivation

Since the middle ages people have been trying to influence the creation of pearls by oysters and mussels. This is named "cultivation" and the pearls that are created in such a way (with some influence by man) are termed "cultured pearls". Often, but not always, the entire product is of natural origin (from mollusks) and by law are considered as natural pearls.
Pearls that grow without any intervenience by men are sometimes named "oriental pearls".

Early cultivation

Although most cultured pearls on todays market are created entirely by mollusks, that was not always the case. In the 13th century [Webster, 1990] the Chinese made blister pearls in the form of Buddha statues by placing a metal Buddha against the innershell of a freshwater mussel and keeping it in place with a metal wire that was connected through an artificial drillhole. Some sources indicate that this has even been done as early as 1082 [Huang et al, 2003].

Carl von Linné (better known as Linnaeus) was able to create the first free cultured pearls in 1761, but the final product was not in total created from mollusks. He drilled a hole in a freshwater mussel and inserted a silver wire with a piece of limestone into the mussel, making sure the limestone was free from the innershell to prevent it from becomming a blister pearl. While he was the first to create some sort of free pearl, the results were not as good as the cultured pearls that are created today.

Modern cultivation

In the late 19th and early 20th century several people were independently researching how to cultivate better round pearls.
One of the most agile and best known of these entrepeneurs was Japanese Kokichi Mikimoto (1858-1954) who was able to create cultured blister pearls in 1893 and was granted a patent for it 2 years later.
Around the same time William Saville-Kent (1842-1908), an English scientist, conducted parallel research on Thursday Island (Torres Strait, Queensland, Australia). Saville-Kent gave evidence of his cultured blister pearls in 1891, 2 years earlier than Mikimoto did [Harrison, 2005].
Both Mikimoto and Saville-Kent were from there on in search of a method to create detached cultured pearls.

The Mise-Nishikawa controversy

In 1907 Japanese Tatsuhei Mise and Tokichi Nishikawa both applied for a patent to create such detached pearls and the method they used is known as the "Mise-Nishikawa method". This method is still used today.

While it took Mikimoto decades of research to create his first detached cultured pearls in 1908, Mise and Nishikawa patented a better method independently of eachother in 1907 with none to little research.
Mise was a carpenter with no training in pearl cultivation and Nishikawa a young graduate from the university of Tokyo. In 1901 Nishikawa and Mise's stepfather (a senior inspector for the Japanese bureau of fisheries) were sent to Thursday Island to inspect the Japanese fleet. It is known that around this time Saville-Kent was experimenting with mantle tissue and nuclei to create round cultured pearls with some success. On the islands in the Torres Strait some 2000 Japanese were involved in the pearl and fishery industry and it is very plausable that Nishikawa and Mise's stepfather have been informed about Saville-Kent's work at Thursday Island. As both Mise and Nishikawa had almost instant success in their efforts to create round cultured pearls instead of doing decades of research, one can only wonder if they indeed were the inventors of their method [Harrison, 2005].

The Mise-Nishikawa method involved a small starting bead (of metal or other material) and some mantle tissue that were placed in the gonad (the reproductive system) of the pearl oyster.

Mikimoto himself was trying to create round pearls in the mantle of the pearl (where natural formation takes place), but his method (which involved silk threads to keep a sack around the starter bead) was not very succesful.
He however perfected the Mise-Nishikawa method and was responsible for the more than succesful marketing of cultured pearls. Due to his never ending diligence cultured pearls are now the norm when we speak of pearls. There is hardly any trade in natural pearls today.

The Mise-Nishikawa method

Cross section of a cultured Tahiti pearl, showing the freshwater nucleus and the natural dark nacre
Photo courtesy of Parels-AEL

This method is the primary method to create round cultured natural saltwater pearls. It involves a starter bead made from the thick shells of North-American freshwater musselshells (such as the pig-toe shell, the three-ridge shell and the washboard shell) and a small tissue of mantle from a donor oyster.

The shells are harvested mainly in the Mississippi and Tennessee rivers. They are sawn into squares and then rounded into spherical beads with a high polish to form a starter bead for saltwater pearl cultivation.
It was Mikimoto who found that starter beads from these shells were best to serve as a "nucleus" for saltwater cultured pearls. We shall refer to the starter beads as nuclei from now on.

As pearls can not grow without a pearl sac, a piece of mantle tissue is taken from a donor oyster and cut in small pieces (2-3 mm).

Young pearl oysters are carefully selected to be implanted with both a nucleus and a small piece of mantle tissue. With high precision an incision is made in the flesh of the oyster to gain access to the gonad and the selected mantle tissue is inserted into the gonad. Then the freshwater nucleus is placed into position in the gonad. It takes great skill to place the nucleus and the mantle tissue so that a pearl sac can be formed from the mantle tissue around the nucleus.
The size and amount of nuclei that can be implanted depends much on the size and species of the oyster.

This all has to be done very rapidly and after the surgery the oysters are placed in special nets which are returned to the sea for a certain amount of time (usually a few years).

Formation of the pearl sac and pearl inside the gonad

The mantle tissue will start to divide its epithelial cells and forms a pearl sac around the nucleus. This sack will then start to deposit conchiolin and nacre onto the nucleus and the pearls will start to grow in concentric rings around that nucleus.

The stages of nucleated cultured pearl formation:

1. The mantle tissue and the nucleus are inserted into the reproductive system of the pearl oyster
2. The epithelial cells of the grafted mantle tissue start to divide
3. The pearl sac is formed
4. The epithelial cells secrete nacre (and conchiolin) onto the nucleus in concentric rings and the pearl grows

Non-nucleated cultivation

A freshwater pearl string (9-9.5 mm) in a plethora of colors
Photo courtesy of Parels-AEL

While the Mise-Nishikawa method involves both a nucleus and a piece of grafted mantle tissue, one can also induce pearl formation without a nucleus. Thus with a grafted piece of mantle tissue alone. This method is used primarely for freshwater pearls.

A small piece of donor tissue is implanted ("grafted") in the mantle of the selected freshwater mussel and this tissue takes nutrition from the mantle in which it is grafted. While the donor tissue grows through cell division it is becoming more and more difficult to get nutrition to the inner epithelial cells and these cells start to decay, forming a cyst with the decayed cell fluids. Epithelial cells that line this cyst now start to secrete layers or conchiolin and nacre and the pearl starts to grow in concentric rings. Up to 40 implantations can be done in a single parent mussel.

The result is a freshwater pearl that is composed of 100% nacre and conchiolin produced by the same mussel, opposed to the nucleated saltwater pearls. In many of these pearls one will find a small void in the center, this is believed to originate from the cyst formation.

In China (where most of this production takes place) they are now able to create large round pearls (upto 15mm) in almost any color, except black. Some of the white freshwater pearls can rival the best of the saltwater Akoya pearls these days [Huang et al, 2003].

It is estimated that these Chinese freshwater pearls make up 90% of the whole cultured pearl production (including saltwater pearls).

Although somewhat round nucleated freshwater pearls were created in Japan (Kasumiga pearls) at some time (and even in the 18th century by Linnaeus), that technique is still not yet perfected in China. It is expected that they will have solved that problem in the foreseeable future.
One can get nucleated freshwater pearls, but not perfectly round ones.

Harvesting

After a certain amount of time the mollusks are taken from the sea, or lake, for the harvesting of pearls.
Depending on whether the mollusk is suitable for another session of cultivation, the mollusks are either broken or are operated to collect the pearls (and in some cases the meat). In the first case the mollusk dies.

After the harvesting of the pearls, they are then cleaned and send to the grading facilities where they are sorted on their physical appearance, like lustre, shape, color, size and blemishes.
Sometimes the harvested pearls undergo a bleaching or staining process to enhance the color.

Grading

Unlike with diamonds and colored gemstones, there is no internationally recognized system for grading pearls. As a consequence there are many systems in use.
What they all have in common is that they take the 6 most important factors in account. They are the 4-S and 2-C factors.

4-S factors

  • Size
  • Shape
  • Shining (orient)
  • Spots (blemishes)

2-C factors

  • Color
  • Coating (thickness of the nacre)

References

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