Pearl June’s Birthstone
CHEMISTRY CaCO3 (aragonite, the outer layer) about 82 – 86%, conchiolin 10 – 14%, water 2%.
CRYSTALLOGRAPHY Aragonite is orthorhombic, with crystals radially oriented.
REFRACTIVE INDEX 1.53 – 1.69
HARDNESS 2.5 – 4.5
SPECIFIC GRAVITY 2.6 – 2.78.
HEAT SENSITIVE Yes
SPECIAL CARE INSTRUCTIONS Avoid heat and all chemicals, including perfume and other cosmetics.
ENHANCEMENTS Dying, common
*Wearability is graded as Excellent, Very Good, Good, Poor, and Forget It! For more details see the article on “Hardness and Wearability.”
Pearls are unique among gemstones, being the only ones found within a living creature and the only ones that requires no fashioning, (cutting or polishing,) before use. Another distinctive feature is its near exclusive use by one gender. Although some efforts have been made to market pearl jewelry to males in recent years, pearls remain the most “feminine” of all gemstones. Designated officially as the June Birthstone, it is, unofficially, a near requirement for brides.
Cultured pearls are those that form in certain mollusks, (oysters and mussels,) at the intervention of man. A piece of mantle tissue, or a shell bead, is inserted into the interior of the animal. This causes it to secrete a layer of organic material, (conchiolin,) over the irritant, followed by layers of nacre (nay-ker.) The composition and structure of this nacre is essentially identical to that which forms under natural conditions. The thin layers of nacre create a kind of diffraction grating through which light must pass and are responsible for the surface iridescence, called orient, so admired in pearls. Pearls have both a body color and “overtones” of rainbow hues created by the orient.
The culturing process takes place over a period of one to three years, depending on the conditions, the species and the desired outcome.
The commercial process for raising freshwater pearls originated in Lake Biwa, Japan at the end of the 1920′s. Various problems, such as pollution and viral diseases, have hampered production in recent years. Progress is being made in restoring the ecosystems and breeding resistant mollusks, so we should see a return of Japanese pearls to a prominent place in the market in the future. At present, however, the premier source is China.
Although once thought of as an inferior product, advances in technique and marketing practices have made today’s Chinese freshwater pearl a true gem. The US continues to command a share of the market with the pearls produced from areas in the South, especially the Tennessee River. Culturing pearls is a delicate process, not assured of success – only 25 – 50% of the altered mollusks produce pearls and generally, only a small percentage of the pearls harvested are of gem quality.
The natural range of colors in freshwater pearls is from white to tan to gray, depending primarily on the species that is used in production. Enhancements are so common that unless it is specifically stated by the seller, you should assume a pearl has been at least bleached to remove dark spots of conchiolin that show through the nacre. More dramatic techniques, such as dying or irradiation, produce pearls with exotic colors such as green, rose and lavender.
Pearls are generally named by their shape; so we have baroques that can be any shape, stick pearls, button pearls, seed or rice pearls, rounds and drops. There are blister pearls, which are created by attaching a bead or other nucleus to the shell of the mollusk and then cutting it out after it has become nacreous. Mabe pearls are assembled from blister pearls, which are filled and glued to a shell base. The term “Keshi” has come to be used for just about any baroque pearl, but in its strictest sense refers to a pearl that spontaneously forms in the oyster during the culturing process, without mantle tissue or bead.
Although pearls are delicate, they have been successfully used in jewelry for thousands of years. As they are sensitive to heat, chemicals and abrasion, they should be stored in a cloth bag or their own box away from contact with other materials. They should be protected from chemicals such as hairspray and perfume. Wiping them with a damp cloth after wearing and occasional cleaning in mild soapy water is all that’s required. Under no circumstances should they be placed in an ultrasonic or steam cleaner. Jewelry settings in rings and bracelets should be protective, or if not, (as in many pearl rings,) the piece should be considered for occasional use only, rather than daily wear.
Faux pearls have been around for a long time and can consist of a variety of materials such as glass, plastic or shell with various surface treatments meant to simulate the pearls luster. With cultured pearl prices at historic lows, there is little incentive to buy or wear imitations. A rule of thumb when testing a suspect pearl is to rub it across the surface of your teeth. Real pearls will feel slightly gritty, most imitatons will feel smooth.
The value of a pearl is most related to the thickness and quality of the nacre. Other factors include size, (especially in rounds,) shape, and color. In general, the highest prices will be paid for large, round, well colored, unenhanced gems. Factors that influence value in pearl jewelry pieces would add to these general considerations, quality of stringing and degree of matching in size and color.
Text and photos courtesy of Barbara Smigel at Artistic Colored Stones.