Pearl

Tahitian and Freshwater Cultured Pearls

Meaning: Pearls (Indian: Moti; Italian: Perla) are classified into two basic categories - 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.

Pearl Properties and Facts

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

Pearl Enhancement

  • 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.

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.

How to take care of pearls?

Precautions which are important since great care must be taken while cleaning pearls.

  • Never clean pearls in an ultrasonic cleaner as it may damage the pearls and wash out the colour if pearls have been dyed.
  • Never clean with steam, strong detergents, bleaches, powdered cleansers, baking soda, or ammonia based cleaners.
  • Do not use toothbrushes, scouring pads, or abrasive materials to clean pearls.
  • Avoid contact with cosmetics, hair sprays and perfumes, since various chemicals present in them may destroy them.

Difference between Pearl and Mother of Pearl 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.
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%