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Pearls

Pearls are known to have been used in jewelry for over 6000 years. 

Physical Characteristics

Hardness: 2.5 - 4.5

S.G: 2.70 (fresh-water up to 2.74)

Size: from microscopic to many centimeter diameter (rare)

Luster - typical pearly luster is termed "orient"

A variety of colors, depending upon the type of mollusc and the water composition (polluted water produces unusual colors)

Body color: underlying color: white-yellow (cream), black

Overtone: "float" (resembles a filmy lacquer): pink / green / blue

Composition:

    • 86 % calcium carbonate (CaCO3)
    • 2 - 4 % water
    • 10 % conchiolin (an organic binding agent)

Together, the conchiolin and CaCO3 are referred to as nacre. Nacre consists of a series of alternating layers of conchiolin and crystals of CaCO3. The CaCO3 is in the crystal form known as aragonite.  The typical iridescence of the pearl is due to the series of nacre layers.  This is referred to as 'orient'(iridescent effect due to overlapping nacreous plates) 

View image of pearls and organic gems

Summary: what makes a pearl a pearl?

  • They must have outer nacre (mostly aragonite) layer to be considered a true pearl,
  • Thus only pearls from mollusks with a nacreous mother of pearl lining are "true" pearls.

Formation

Pearls are produced by a variety of organisms, commonly sea molluscs. They are also produced by fresh water mussels, and occasionally, by snails.   Some examples of pearl-producing oysters (you don't have to remember these) are:

  • Akoya pearl oysters (Pinctada fucata)
  • Black Lip Pearl shell (Pinctada margaritifera)
  • Freshwater mussel (Hydriopsis schlegeli)
  • Large winged pearl shell (Pteria penguin)
  • Abalone (Notohaliotis discus)
  • Golden Lip pearl shell or white lip pearl shell (Pinctada maxima)

Natural Pearls:

Concentric layers of CaCO3 are deposited around an irritant. This may be a piece of mantle lobe or some other material. Only the mantle lobe can secrete nacre. When a piece of mantle lobe is introduced by some accident into the tissue of the oyster, the oyster forms a bag known as a "pearl sac". It is this sac that secretes the nacre around the irritant to make the pearl.

Thus, pearls are calcareous concretions. Some natural pearls have quite unusual shapes. These are often called "baroque" pearls.

Both saltwater and freshwater pearls consist of the same material and can form in "baroque" shapes. Unless you are quite familiar with the typical characteristics ("look") of pearls from certain specific sources, it would be very difficult to know whether a given pearl was saltwater or freshwater in origin.

Probably the most common freshwater pearl on the market is the Chinese freshwater baroque, some of which are crinkily and look like crisped rice. These have been very popular in recent years because they cost dramatically less than Akoya cultured pearls.

Blister Pearls:

Blister pearls form on the inside of the mother of pearl shell.

 

Cultured Pearls: 

Oysters and mussels are induced to make pearls. The result are termed "cultured pearls". Maybe 90% of the pearls sold are cultured.

If you break a pearl open you will see that it consists of a bead covered by a thin layer of nacre. The culturing process involves inserting a small piece of mantle lobe and a bead made from mother of pearl shell into the tissues of a pearl-producing mollusk.

 The mollusk treats the bead as an irritant and the mantle lobe tissue begins to deposit a nacreous coating over it.

Here is a description and some photographs to illustrate the process. The photographs were taken at the Mikimoto Pearl Museum, Toba, Japan:

·  Oysters are raised in a tank, allowed to attach to fibers, then grown in sea water for two to three years. Growing oysters are suspended in cages hung from rafts. They feed on plankton. Healthy oysters are selected for pearl cultivation.

·  The bead is prepared. Mikimoto use Pig toe clam shells, from the Mississippi River. Small balls are prepared from pieces of these shells. An example of amother of pearl bead.

·  Living oysters are wedged open and a piece of mantle lobe harvested from an other oyster, plus a bead, are inserted into the soft tissue. This image shows insertion of mantle tissue and bead. Here is a labelled version of this image, showing the important components.

·  This image shows the oyster source of mantle tissue, the cut up pieces of mantle tissue, and the mother of pearl beads. A labelled version of this image is given here.

·  Oysters are then returned to the sea, where they are suspended in cages 7 - 10 feet below the surface. They are maintained and harvested after some time. The culture period used to be ~ 3.5 yrs, producing ~ 1mm layer on the bead, but now the culture period may take less than 2 yrs.

The commercial production method is now known as the Mise-Nishikawa method.

 

 

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