Natural & Artificial Pearls: Composition & Chemistry
A pearl is a hard, rounded object produced by certain mollusks, such as oysters. The pearl is a valued gemstone and is cultivated or harvested for jewelry. Natural pearls are formed when a small foreign object, such as a parasite, grain of sand, or piece of food, lodges itself in the gonad or mantle tissue of a mollusk or oyster. When the mollusk is invaded by a foreign object that the animal cannot eject, a process known as encystation entombs the offending entity in successive, concentric layers of 'nacre' (nay'ker). In a defensive response to the irritant, the mollusk secretes nacre as a smooth, protective coating.
Pearl Mollusk Types (Illustrations: Public Domain)
As the nacre builds up around the irritant, it forms layers, eventually creating a pearl. Natural pearls that are both large in size (diameter), and have a very symmetrical shape are extremely rare. For pearls to form in nature it can take may years of near-perfect conditions for them to make a significant gain in size.
The principle difference between "natural pearls" and "cultured pearls" is the thickness of the nacre. Since natural pearls take longer to develop, there is generally a thicker layer of nacre surrounding the nucleus. It can take as long as two to five years for a quality pearl to fully develop in the oyster. Many lower quality cultured pearls are created by inserting a large nucleus and hastening the process of nacre development. This results in a pearl with a very thin layer of nacre that will not have a great deal of luster, and will not be very durable over a long period.
Pearls are formed inside the shell of certain bivalve mollusks. Nacre, also known as mother-of-pearl, is a naturally-occurring organic/inorganic composite, that is a combination of crystalline and organic substances that form the iridescent inner lining of the shell.
In certain species of mollusk, nacre is secreted by specialized epithelial cells located close to the shell margin, in the outer mantle fold. Along the inner surface of the fold, near the periostracal groove, the stratified columnar epithelial cells occur. In these mollusks, nacre is continually deposited onto the inner surface of the animal's shell. This process creates the iridescent nacreous layer, commonly known as mother of pearl.
The nacre is produced as a means of smoothing the internal wall of the shell and as a defense against parasitic organisms and damaging detritus. When a mollusk is invaded by a parasite or is irritated by a foreign object (nucleus - photo to right) that the animal cannot eject, a process known as encystation entombs the offending entity in successive, concentric layers of nacre. As a response to an irritating object inside its shell, the mollusk will deposit layers of calcium carbonate held together by an organic horn-like compound called conchiolin. This combination of calcium carbonate and conchiolin is called nacre or mother-of-pearl.
About 95% of nacre consist of layers of calcium carbonate (CaCO3 - photo to left) platelets or crystals, a weak and brittle ceramic in the form of aragonite and conchiolin (a scleroprotein), separated by elastic biopolymers (such as chitin and lustrin). Both components are rather weak materials when separated. This mixture of brittle nanometer platelets and the thin layers of elastic biopolymers makes the material strong and resilient. Strength and resilience are also likely to be due to adhesion by the "brickwork-like" arrangement of the platelets, which inhibits transverse crack propagation.
Calcium Carbonate & Nacre (Photos: Public Domain)
By x-raying the pearl (photo above/right), jewelers can determine whether it has been cultivated or is natural. If the nucleus in the center of the pearl is a perfectly round sphere, the jeweler will know that it has been "cultivated". When most pearl cultivators insert the nucleus or "grit", it is usually pefectly round, so as to produce a more valuable, round pearl. If the center is not perfectly round, the jeweler will conclude that the pearl is a genuine "natural" pearl, and give it a higher value. The bead nucleus of a cultured pearl is denser and thus less transparent to X-rays than the surrounding nacre and conchiolin. A natural saltwater pearl will show concentric rings of alternating layers of conchiolin and nacre that are similar to the growth rings of a tree cross-section.
Iridescence in Pearls (Photos: Public Domain)
The iridescent quality of nacre (or mother-of-pearl) is due to the fact that the thickness of the aragonite platelets are about 0.5 micrometres, which is comparable to the wavelength of visible light. This results in absorptive and reflective effects on different wavelengths of light (pseudochromatic coloration), resulting in different colors of light being reflected when observed at different viewing angles.
Conchiolin and perlucin are complex proteins which are secreted by a mollusk's outer epithelium (or the mantle) to form chambers. These chambers hold and bond to the crystals of aragonite, giving the mollusk's shell its stiffness (image to left). Conchiolin is sometimes referred to as conchin. The ions necessary to form calcium carbonate are also secreted by the mantle, but it is the ionic environment of the perlucin protein which causes aragonite rather than calcite crystals to nucleate.
Pearls Conchiolin (Photos: Public Domain)
The conchiolin serves as a relatively flexible, crack-deflecting matrix for the mineral aggregate particles. Conchiolin's strength and the strong bonding of perlucin give the finished nacre an impressive level of strength and durability.
The unique lustre of pearls depends upon the reflection and refraction of light from the translucent layers of the nacre. The iridescence that some pearls display is caused by the overlapping of successive layers, thus breaking up the light falling on, and reflecting from the surface (scattering). Pearls are usually white, sometimes with a creamy or pinkish tinge, but may be tinted with yellow, green, blue, brown, purple, or black.
Black pearls, frequently referred to as Black Tahitian Pearls, are highly valued because they are rare. Black Tahitian pearls cannot be mass produced because of the fragile health of the oyster, non-survival of the implantation process, rejection of the nucleus, and the oyster's sensitivity to changing climatic and ocean conditions.
Bibliography & Suggestions for Further Study on Pearls
1. Pearl Guide, Japanese Akoya Pearls . www.pearl-guide.com
2. K Mikimoto & Co, Ltd, Pearls . www.mikimoto.com
3. Seikai National Fisheries, Pinctada margaritifera Cultured. www.lib.noaa.gov
4. American Museum of Natural History, Pearls . www.amnh.org
5. U.N. Food & Agriculture Org., Pearl Oyster Taxonomy & Distribution . www.fao.org