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!)
• bodycolor: 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 irridescence of the pearl is due to the series of nacre layers.  This is referred to as 'orient'(iridescent effect due to overlapping nacreous plates)

• 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

History of culturing of pearls:
Care of Pearls!
How to buy pearls
 

Formation

• 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)

Reference: Mikimoto Pearl Museum, Toba.

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 form on the inside of the mother of pearl shell

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 a mother 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

Typical results:

 (you do not need to memorize these numbers)

5% high quality pearls (hanadama)

28% marketable pearls

17% unmarketable pearls

5% uncoated nuclei

50% of oysters containing nuclei will die.

Source: Mikimoto.

Selection and presentation of pearls: Pearls are selected for their size and color (hue). Careful color grading is extremely important.

Pearls are then drilled from both sides, often at a place that is slightly flawed. They are then sorted, threated, and marketed.

Mabe pearls are cultured blister pearls. These are produced by inserting a half bead against the shell of the mollusk, after a layer of nacre has been deposited over the bead, the whole formation is cut out and the nacreous dome cemented onto a mother of pearl bed.

Biwa pearls are produced at lake Biwa, Japan using freshwater clams. They are irregular in shape but have good color and luster. Instead of a bead a small square of mother of pearl in inserted into the clam. These pearls require three years to produce good results.

Natural or Cultured?

The term "candling" refers to examination of pearls in strong light. This may reveal the mother of pearl bead.

If the pearl is undrilled, an excellent method to distinguish cultured from natural pearls is to X-ray them.

Because the size of the seed differs, natural pearls ar less dense than most cultured pearls.

Information comparing Japanese and Chinese pearls
 

Use

Preservation and damage

Pearls are damaged by excess humidty, dryness, acids, perspiration, cosmetics, hair sprays, and other chemicals.
 

Treatments

Conch "pearls": because they lack nacre, these are not considered real pearls. They are often orange or pink in color. They form as concretions in conch shells.