Pearl

Pearl (Indian: Moti; Italian: Perla) is classified into two basic category - Natural and Cultured. The term synthetic must not be used with respect to pearls. Materials which simulate or look like pearls, but do not have either the composition or structure, are known as imitation pearls.

 

  • Natural Pearl
    • Saltwater Pearl
      • Persian Gulf
      • Australia
      • Japan
      • Straits of Manaar (Region between India & Sri Lanka)
      • South Sea Islands
    • Freshwater Pearl
      • Mississippi (U.S.A.)
      • China
  • Cultured Pearl
    • Saltwater Pearl
      • Japan
      • Australia
      • Philippines
      • South Sea Islands
    • Freshwater Pearl
      • Lake Biwa (Japan)
      • China
  • Imitation Pearl are products, completely or partially man-made, imitating the appearance, colour and effect of natural or cultured pearls without possessing the physical and chemical properties, even when using natural substances.

Information on Pearls

Pearls are produce by molluscs which are saltwater oysters and freshwater mussels.

The oyster consists of a soft visceral mass enclosed between two halves of the shell which are hinged together. Like every other animal, an oyster possesses a heart, stomach and mouth. It breathes through its gills and feeds itself with micro-organisms and planktons in the water.

  • Pearl Oysters belong to the Pinctada family of the mollusc group in class Lamellibranchia. Pinctada molluscs are saltwater oysters. Molluscs like Unio and Hyriopsis are freshwater mussels.
  • The shell is composed of various layers.
  • The first layer is dark horny outside layer composed of organic substance conchiolin, a protein like substance.
  • It is also the binding agent that holds the aragonite crystals together.
  • The inner layer is a thicker crystalline layer of prismatic calcium carbonate in the form of calcite.
  • Mother of Pearl is the smooth pearly lining (layer) on the interior of a mollusc shell. It is composed of CaCO3 as aragonite crystals and a little water and conchiolin.
  • Aragonite layer is made up of microscopic platy calcium carbonate crystals arranged in an overlapping manner.
  • Nacre is a protective pearly substance secreted by the mollusc and deposited around an irritant, in layers. These layers decide the lustre and quality of a pearl. The thinner the layer, the better is the lustre and vice versa.
  • Mantle is a fold of epithelial material that envelopes the internal organs of the animal and is situated at the meeting point of the inner sides of the shell. It secretes conchiolin, aragonite and calcite.
  • Orient of a pearl results from the diffraction of the light through the aragonite crystals.
  • When white light is reflected from two different surfaces very close together, the two reflected waves can be out of phase, which causes interference, resulting in rainbow colours.
  • The finer the pearly layers, the greater the orient that the pearl will possess.

Pearls Produced by Snails

  • Abalone Pearls: Found off the coast of New Zealand, California, Mexico, Japan and Korea. These pearls are known for their almost opal like appearance. The colours may be any combination of green, blue, pink or yellow and usually have an irregular shape.
  • Conch Pearl: These pearls are found in the great Conch, a large marine snail found throughout the Caribbean. Commonly pink, white or brownish. They are also called 'pink pearls' and may have distinctive flame like surface markings. These pearls do not have a nacreous coating and appear more as corals.

Abalone Pearl Pictures

Abalone seashell Abalone with pearls

Difference between Pearl and Mother of Pearl

Difference is minor and varies in their structures and the proportions of their components.

  • Comparing a pearl and a ball of mother-of-pearl, it will be seen that the pearl has lustre over its entire surface, whereas the ball of mother-of-pearl has this in only two places. This is because their structures are totally different.
  • The mother-of-pearl has relatively flat layers, whereas the pearl has been formed around a nucleus in concentric layers.
  • The components of pearls or mother of pearl are affected by the food, the salinity, the water temperature and the region where the oyster-beds are found.
Composition Pearl Mother of Pearl
Calcium Carbonate (Aragonite) 91.72% 84.75%
Organic Material (Conchiolin) 5.94% 11.76%
Water 2.23% 3.17%
Other Substances 0.11% 0.50%

Pearl Pictures

Pearl in various color Black pearl in an oyster shell

Pearl Properties

Chemical Composition: 
Calcium carbonate, organic material and water
Classification / Type: 
On the basis of shape, body color, overtone and source.
Colors / Varieties: 
All colors / Orient and Sheen.
Crystal System / Forms: 
None. Partially crystalline and partially amorphous.
Hardness: 
2.5 - 3.5
Specific Gravity: 
  • Black pearls: 2.20 - 2.66
  • 2.67 - 2.78
Cleavage / Fracture: 
None / Uneven
Optic Character: 
A.G.G.
Lustre: 
Varies from almost dull to a near metallic in black pearls; orient may be sheen.
Refractive Index / Birefringence: 
1.530 - 1.686 (Spot reading: 1.55) / 0.156
Pleochroism: 
None.
Dispersion: 
None.
Magnification: 
Drill hole might show the layered internal structure, presence of dye.
U.V. Fluorescence: 
Variable; inert to blue-white in both longwave and shortwave. Inert to x-rays.
Spectrum: 
None.
Specific Tests: 
  • Drill hole test; candling
  • X-ray diffraction / radiography
  • Endoscopy
  • Lucidoscope
Synthesis: 
None.
Simulants (with separation tests): 
Geological Occurrence: 
Inside of a living organism namely oysters or mollusks.
Sources: 
Persian Gulf, India, Sri Lanka, Philippines, Malaysia, Australia, Tahiti, Mexico, Panama, Venezuela etc.
Cuts & Uses: 
As cabochons, beads, carving, etc.

Pearl Treatments

  • Skinning: It is performed by removing a bad coloured or blemished outer layer, so that a more attractive, although smaller, pearl could be obtained. The layer is removed by careful filing or by the use of abrasive emery paper.
  • Soaking: Cracks in the surface of pearls are sometimes cured by soaking them in warm olive oil.
  • Coloured Impregnation (Dyeing): Staining is commonly done on Akoya pearls. The pearls are soaked in a weak solution of silver nitrate and dilute ammonia and then exposed to light or hydrogen sulphide gas. Occasionally the core (bead nuclei) is dyed before they are inserted in the oyster to produce a different overall colour. It is performed on white pearls to change them into black pearls.
  • Coating: Pearls may be coated with any pigment to give different colours.
  • Bleaching: Removal of a secondary colour or stains from the deposit of organic matter. This is done by immersing the pearls in a solution of oxygenated water and afterwards exposing them to light.
  • Irradiation: Some pearls are darkened by treating them with gamma rays. Commonly Akoya pearls are treated to various coloured shades. X-rays may give a reduced silver shade.

Most pearl enhancements are difficult to detect with conventional equipment. Enhancements such as skinning and bleaching are often considered as part of the processing of pearls by traders.