Pearls are unique among gemstones. The only gem material found within a living creature, they require no cutting or polishing before use. Although oysters are their best-known source, clams, mussels, and abalone produce as well. Natural pearls are extremely rare today, with only one in several million shellfish ever yielding a pearl. Cultured or farm-grown pearls, however, are common. Pearls are also one of our most ancient gems. They've been prized as jewelry for 6,000 years, and records of their commercial harvesting date back at least 2,500 years. (Their cultivation dates back to at least the 13th century CE in China). Wearing, storing, and cleaning pearls all require special care. Nevertheless, the appeal of the traditional June birthstone as a jewelry piece remains undiminished.
The most important factor in grading a pearl is if it's natural or cultured. Other factors are luster, shape, surface condition, color, and size.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Varieties||Conch Pearl, Freshwater Pearl, Saltwater Pearl|
|Crystallography||Amorphous. The aragonite in the nacre of a pearl is orthorhombic, with minute crystals radially oriented and a concentric structure.|
|Colors||Pearl color is the result of a body color and an overtone color or orient present as a lustrous sheen. The orient is the color seen as reflected by a diffuse light source. The rest of the color is due to the body color. There are sometimes two overtone colors, one seen on the surface in full view, the other at the edge. See “Identifying Characteristics” below for more information.|
|Polish Luster||Dull to nearly metallic.|
|Fracture Luster||Dull to pearly.|
|Fracture||Uneven. Roughness is variable.|
|Hardness||2.5 - 4.5|
|Specific Gravity||2.6 – 2.78; conch pearls, 2.85; cultured pearls, 2.72 - 2.78, that is, heavier than most naturals. However, this is NOT a diagnostic test.|
|Birefringence||0.155 – 0.156 (aragonite)|
|Luminescence||See “Identifying Characteristics”|
|Wearability||Good to poor.|
|Transparency||Translucent to opaque.|
|Formula||CaCO3 (aragonite, the outer layer) about 82-86%, conchiolin 10-14%, water 2%. These proportions are variable.|
|Optics||Aggregate, if not opaque. N = 1.53-1.69, but not observed; usually vague shadow edge in this range|
|Etymology||From the Old French perle, Medieval Latin perla, and Classical Latin pernula or perna for pearl.|
|Occurrence||Formed within various species of living bivalve mollusks in both salt and freshwater.|
For a pearl to form, an irritant (usually a grain of sand) must get deep enough inside of the shell that the mollusk cannot expel it. As a result, the creature’s shell producing system begins coating the irritant with nacre (NAY-ker), the shiny substance found on the interior of most shells. Nacre is composed of the mineral aragonite with an organic binder called conchiolin. The thin layers of nacre create a kind of diffraction grating through which light must pass.
Despite efforts to market pearl jewelry to men in recent years, pearls remain the most “feminine” of all gemstones. Pearl has another, unofficial, June association: brides. This isn’t just the result of clever modern marketing. Pearls have been connected to weddings, fertility, and love in India for thousands of years.
Natural pearls almost disappeared from the market in the late 1800s due to over-harvesting. Even today, they’re extremely rare and can command a ransom. As the natural sources were exhausted, the modern era of cultured pearl production began in Japan around 1910. “Perliculture” or pearl farming make it possible to produce pearls in greater quantities and larger sizes than can grow in nature.
Diffraction at the edges of overlapping plates of aragonite crystals cause the surface iridescence or orient of pearls. (These edges also cause the roughness felt in a “Tooth Test”). A pearl has both a body color and “overtones” of rainbow hues created by this much-admired phenomenal effect.
- White: white with no overtone; cream with no overtone, light cream to light yellow; light rose (pinkish overtone on white background); cream rosé (cream background with deep rose overtone); fancy pearls (cream background with overtone of rose; blue or green secondary overtone seen at edges of pearl).
- Black: includes gray, bronze, dark blue, blue-green, and green. Some have metallic overtones.
- Colored Pearls: neither black nor white, usually with a blue background color, plus red, purple, yellowish, violet, blue, or green. More frequently seen in freshwater pearls.
Darker colors are apparently due to dark conchiolin in the core of a pearl showing through the thin layers of aragonite crystals.
Natural pearls may be inert to strong light blue, yellowish, greenish, or pinkish in both longwave (LW) and shortwave (SW) ultraviolet light (UV). Cultured pearls may have no reaction or the same reaction as naturals in LW. Freshwater pearls always glow yellowish white in X-rays.
Natural black pearls may be weak to moderate red, orangish red, or brownish red in LW. Dyed black pearls may show variable reactions under LW but never the same as natural.
La Paz pearls have a strong red reaction in LW.
Cultured is the equivalent of “synthetic” in the world of pearls. Cultured pearls are formed under the same conditions as their natural counterparts and are chemically and physically the same as the natural products, with a few tell-tale differences. However, cultured pearls are created in salt and freshwater farms instead of laboratories.
In underwater pearl farms, the cultivators carefully insert pieces of mantle tissue and mother-of-pearl seeds or shell beads into the interiors of bivalve mollusks, such as oysters. The animals secrete nacre to coat the irritants, just as they would in nature. The composition and structure of this nacre is essentially identical to that which forms naturally.
After this “surgery,” the oysters convalesce in a “hospital” for four to six weeks. They are then transferred to cages between seven and ten feet under water. Here, they are allowed to grow for one to six years. The cultivation period depends on the farm conditions, the mollusk species, and the desired pearl result.
Cultured pearls can be distinguished from naturals by an X-ray examination at a gemological laboratory. This can reveal the seed used to start the formation of nacre layers. (It might also be possible to see the seed if a pearl has a drill hole).
Faux or fake pearls are simulants. While cultured pearls are real pearls, simulants are not pearls at all. With various surface treatments, materials such as glass, shell, and plastic can be made to imitate a pearl’s luster. Faux pearls have been around for a long time. However, with cultured pearl prices at historic lows, there is little incentive to buy or wear imitations.
To test whether a pearl is real or fake, try the “Tooth Test.” Rub the pearl across the surface of your teeth. Real pearls will feel slightly gritty or rough. Most imitations will feel smooth.
Various treatments can change the color of pearls.
- Bleaching to remove black conchiolin is common. This is a stable and undetectable treatment.
- Dyeing is also common. Signs of this treatment can be detected around a pearl’s drill hole.
- Gamma radiation can turn pearls a grey to blue-grey color and improve the color of greenish pearls.
Round pearls may range in size from 2 to 9 mm. Baroque pearls may reach 50 mm.
For large and famous named pearls, see the listings for freshwater pearls and saltwater pearls.
Pearls can be found in symmetrical round and pear shapes. Squat pear shapes are referred to as egg shapes. Elongated pear shapes are referred to as drop shapes. Pearls are generally named after their shape. In addition to rounds and drops, you can encounter stick pearls, button pearls, seed or rice pearls, etc. Baroques can be called many things.
- Baroque refers to any irregular shaped pearl.
- Button pearls are round with flat backs.
- Blister or mabe pearls aren’t true pearls. They are dome shapes, originally attached to the inside of the shell.
- Bombay refers to pearls with a cream body color with a rose overtone.
- Ceylon or Madras are white or cream pearls with fancy overtones of green, blue, or purple.
- Colored pearls have a pronounced body color.
- Conch pearls aren’t true pearls, although they always carry this name. See the calcareous concretions listing for more information.
- Dust pearls are too small for use in jewelry. Usually, they’re less than 1 mm or 1/25 grains.
- Fancy pearls have two overtones, one rose, the other green or blue.
- French pearl is a misnomer for a piece of shell.
- Half or ¾ pearls have flat backs and are usually drilled.
- Hinge pearls are the actual hinges of the shell. They’re not true pearls.
- Keshi is sometimes used to refer to almost any baroque pearl. However, in its strictest sense, it refers to pearls that form spontaneously during the culturing of South Sea pearls. They are all-nacre, non-nucleated pearls. They are essentially natural, though they’re found in farmed beds.
- La Paz pearls are from the coast off Baja California, Mexico.
- Mabe are blister pearls that are filled and glued to a shell backing.
- Maiden pearls are newly harvested.
- Mother of Pearl is the iridescent inner layer found in most seashells.
- Oriental pearls are natural pearls from the Persian Gulf.
- Osmenda pearls aren’t pearls but cabochons cut from a chambered nautilus shell.
- Pink refers to any white or cream colored pearl with a rose overtone.
- Slugs are baroque pearls with poor luster.
- Seed pearls are less than 2mm or 1/4 grain. They’re usually asymmetrical or off-round in shape.
- Tridacna pearls aren’t true pearls, although they always carry this name. See the calcareous concretions listing for more information.
Although pearls are delicate, jewelry makers have successfully made wonderful pieces from them for thousands of years. Nevertheless, if you want to enjoy your pearl jewelry for even a fraction of that time, you should exercise some caution. Along with opals, pearls merit their own detailed care guide.
Here are some basic guidelines:
- Store pearls in a cloth bag or in a box away from other gems. Most gems commonly found in jewelry collections can scratch pearls because they have such low hardness.
- Always put on your pearl jewelry last, after you’ve put on perfumes and hairspray. These products may contain acids and alcohols that can ruin pearls.
- Wipe your pearls with a damp cloth after wear.
- Never submerge your pearl jewelry in soapy water. Wipe it with a damp cloth and mild soapy water (not detergent).
- Never use mechanical systems like ultrasonic or steam to clean your pearls. They’re very heat sensitive.
- Pearl rings and bracelets should have protective settings. If they don’t, wear them only occasionally rather than daily.