PEARLS AND OTHER ORGANIC GEMS

PEARLS

Pearls are one of the oldest known gems: (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 diamter (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!)
  1. bodycolor: underlying color: white-yellow (cream), black
  2. overtone: "float" (resembles a filmy lacquer): pink / green / blue
• composition:
  1. ~ 86 % CaCO3
  2. 2 - 4 % water
  3. ~ 10 % conchiolin (an organic binding agent)
     • gaps in conchiolin fill with aragonite (CaCO3) -> nacre.
• 'orient': iridescent effect due to overlapping nacreous layers (plates)

Crystals of CaCO3 are deposited around an irritant such as a grain of sand or piece of shell

• Blister pearls form on the inside of the shell
• some natural pearls have quite unusual shapes These are sometimes termed "baroque" pearls.

1. Natural:
   mollusks deposit crystals of CaCO3 around an irritant or parasite. Thus, pearls are calcareous concretions
   • they must have outer nacre (mostly aragonite) layer to be considered a true pearl

   • thus pearls only from mollusks with a nacreous mother of pearl lining are "true" pearls

   • Naturally occuring and cultured freshwater and saltwater pearls both consist of the same material, nacre (deposited by the mollusk over a nucleus). What's more, both saltwater and freshwater pearls can form in all manner of baroque shapes. So 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.
     • salt water oysters
     • freshwater mussels

   • Probably the most common freshwater pearl on the market is the Chinese freshwater baroque, including the little crinkily ones that look like strands of crisped rice. These have been very popular in recent years because they cost dramatically less than Akoya cultured pearls.

2. Cultured Pearls: (maybe 90% of the pearls sold are cultured)

   A cultured pearl is produced by inserting a mother of pearl bead into the tissues of a pearl-producing mollusk.

   The mollusk treats the bead as an irritant and deposits a nacreous coating over it.

   Here are some examples from the Mikimoto demonstrations, Toba, Japan:

3. insertion of mantle tissue and bead MABE pearls (cultured blister pearls) 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.

   History of culturing of pearls:

   commercial production is now by the Mise-Nishikawa method saltwater: a mother of pearl bead and a piece of mantle tissue is inserted into an incision in the foot of the mollusk
   oysters are held in cages under rafts ~ 7 - 10 " deep.
   culture period used to be ~ 3.5 yrs, producing ~ 1mm layer on the bead, but now the culture period takes less than 2 yrs

Natural or Cultured?

Distinction between these can often be made if the pearl is drilled (the seed can be seen a mm or so beneath the surface).

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

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

Use

The majority are strung as necklaces. Some are used in rings

Preservation and damage

Conchiolin is prone to drying. If this occurs, the pearl becomes dull, the surface cracks and finally peels.

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

Treatments

Pearls are sometimes bleached to lighten their color

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How to buy pearls!....Care of Pearls!

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Conch Pearls

"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.

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Other organic gems

Coral, amber (which may include a variety of inclusions and cracks) and the tagua nut, which is used in place of ivory

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Questions on the lecture

View some images of pearls and organic gems

" Precious Stones: Agate and Opal"

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