Pearls are one of our most ancient gem materials. They’ve been prized as jewelry for 6,000 years. Records of their commercial harvesting go back at least 2,500 years, and the cultivation of so-called “blister pearls” dates back to at least the 13th century CE in China.
Crotalia earrings like these were popular pieces of jewelry in Imperial Roman times because the pearl pendants jingled when worn. Gold and pearls. Roman, 1st century CE, 2.9 cm. Rogers Fund, 1920. Metropolitan Museum of Art, New York. Public Domain. (Cropped to show detail).
Although oysters are their best-known source, other mollusks such as clams, mussels, and abalone produce them as well. Today, natural pearls are extremely rare. In the wild, only one in several million of these creatures will ever yield a pearl. However, cultured or farm-grown pearls are common.
How do Pearls Form?
For a pearl to form, an irritant 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. This creates a pearl’s distinctive surface luster.
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 natural sources reached exhaustion, the modern era of cultured pearl production began in Japan around 1910. “Perliculture” or pearl farming makes it possible to produce pearls in greater quantities and larger sizes than can grow in nature.
Despite the addition of alexandrite to modern lists, pearl remains the gemstone most commonly known as the June birthstone. Pearl has another, unofficial, June association: weddings and, in particular, brides. This isn’t just the result of clever modern marketing. Pearls have been connected to weddings, fertility, and love for thousands of years.
For more information on pearl symbolism, see our article on its legends and lore.
Aquamarine and 7 mm white pearl in a heart pendant, whimsically referencing the parable of the pearl. © CustomMade. Used with permission.
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.
Pearl: Worldwide localities, selected to show color variation. Photo © Joel E. Arem, PhD, FGA. Used with permission.
- 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; also, blue or green secondary overtone seen at edges of pearl). Freshwater pearls are more likely to have fancy colors than saltwater pearls.
- Pink (refers to any white or cream colored pearl with a rose overtone).
Black pearls derive their color from a layer of dark conchiolin showing through the nacre. Their body colors include black, gray, bronze, dark blue, blue-green, and green. Some have metallic overtones.
These pearls have a pronounced body color, any color except black or white. The most common body colors include blue, red, purple, violet, yellowish, and green. Colored pearls usually have a blue background color. You’ll find these colors more frequently 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.
The fluorescence of natural pearls may range from 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.
What are Cultured Pearls?
The equivalent of synthetic gems 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 continue 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, so use your judgement before having an X-ray test).
Faux or fake pearls are simulants. While cultured pearls are real pearls, simulants aren’t 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’s 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. Look for signs of this treatment around a pearl’s drill hole. Dyes are usually visible through the drill hole as a color layer under the nacre. See our article on dye testing for more information.
- Gamma radiation can turn pearls a grey to blue-grey color and improve the color of greenish pearls.
Earrings with cranberry red-dyed freshwater pearls on fine silver headpins and sterling silver earwires. Photo by Leah. Licensed under CC By 2.0.
The more light a pearl’s surface reflects and the sharper its reflections, the higher its luster. A very high or high luster means the reflections are bright and sharp. A soft or dull luster means the reflections are weak, fuzzy, and diffused.
The thickness of a pearl’s nacre layers affects its durability and relates directly to its orient and luster. Professionals consider a thickness of 0.25 to 0.35 mm good commercial grade, while 0.35 to 0.5 mm represents gem grade. However, problems with pearl cultivation usually lead to faster harvests, which affects nacre thickness. As a result, the highest grades now usually top out around 0.4 mm.
While measuring nacre isn’t always possible, you can get a good idea of its thickness by the quality of a pearl’s luster and orient.
The closer a pearl comes to round, the higher its value. Symmetry is a secondary factor. Typically, an oval will command more value than a baroque shape.
Pendant designed around a set of family gems we removed from this customer’s heirloom jewelry. Created a new fleur-de-lis pendant in 14k yellow gold, studded with diamonds, with a large, dangling South Sea pearl. © CustomMade. Used with permission.
Round pearls are almost perfect spheres with less than 2% variation in diameter. To determine whether a pearl is round, a professional tester rolls it across a table. If it rolls straight, it’s round. If the pearl veers off more than slightly, it’s semi-round. Semi-round pearls (also called off-round or near-round) are slightly imperfect spheres. Their diameter variation rate falls between 2% and 5%.
Charm/pendant in gold of an angel holding a pearl to symbolize the moon. Designed as a memorial piece in honor of the recipient’s deceased mother, who would tell her daughter to look at the moon and know that they were looking at the same moon. © CustomMade. Used with permission.
Ringed or circled pearls show regular streaks, rings, or grooves. These markings must be perpendicular to the axis of rotation and cover more than a third of the surface.
Baroque pearls are irregular in every direction. They don’t have an axis of rotation. However, semi-baroque pearls exhibit at least one axis of rotation. That means they can spin on one end like a top. Subdivisions of baroques include drop, button, pear, and oval shapes.
Surface grading is done with the naked eye rather than a loupe. Two principal factors influence the surface grade: the size or texture of the surface irregularities and how much surface area they cover.
The Tahitian pearl grading system uses the following grades. However, the principles apply to all pearls.
- Quality A pearls: entirely smooth. A- pearls have just one or two tiny ripples or indentations, confined to less than 10% of the surface. A drill hole will usually hide these indentations later.
- Quality B pearls: lightly blemished. They have minor imperfections covering less than a third of their surface.
- Quality C pearls: moderately blemished. They have light imperfections covering less than two-thirds of their surface.
- Quality D pearls: heavily blemished. These pearls have light imperfections over more than two-thirds of their surface and no deep imperfections, or deep imperfections over half of their surface.
When color grading pearls, the body color and overtone need to be considered simultaneously. Not all pearls have overtone and it varies greatly in those that do.
In light pearls, body color values go to white, then silver, next to cream, and then into the progressively more yellow shades. With the addition of orient, top values go to light rose, white body color with pink overtones. Next comes cream rose, pearls with a cream body color with a deep rose overtone.
The most valuable fancy pearls have cream body colors, with rose overtone and green or blue as a secondary overtone.
The darker and closer to black, the higher the value of black pearls. Top-grade black pearls have a green overtone called “peacock green.” Multiple colors of overtone come next in value.
See the gem listings for freshwater pearls and saltwater pearls for information on pearl-producing mollusk species and natural and cultivation sources.
Pearls are measured in millimeters, grains, and mommes. A jewelers grain is equal to 0.25 carats. A momme is used for quantities of pearls. One momme is 75 jewelers grains, or 18.75 carats. (Note that a grain has different values in other measuring systems).
Round and off-round pearls are measured by their shortest diameter. Pearls in other shapes are measured along the longest and second longest dimensions.
As with any other gem, larger specimens are less common, hence they demand a higher price per weight. 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 develop in symmetrical round and pear shapes. Squat pear shapes are referred to as egg shapes, while elongated pear shapes are referred to as drop shapes.
Pearls are generally named after their shape. In addition to rounds and drops, you may come across stick pearls, button pearls, seed or rice pearls, etc.
Baroque refers to any irregularly shaped pearl. These pearls have many different trade names. Although symmetrical spheres may be the most popular and traditional pearl shape, the many unusual forms baroques may take can inspire some unique jewelry.
Note how the baroque pearl’s irregular shape captures the form of a torso from both front and back. Pendant in the form of a siren, baroque pearl with enameled gold mounts set with rubies. Probably ca. 1860, European, height 10.6 cm. The Jack and Belle Linsky Collection, 1982. Metropolitan Museum of Art, New York. Public Domain. (Cropped to show detail).
Baroque pearls with poor luster are called slugs.
Sometimes, the name keshi is used to refer to almost any baroque pearl. However, strictly speaking, the term refers to all-nacre, non-nucleated pearls that form spontaneously during the culturing of South Sea pearls. Thus, though they form in farmed beds, they’re essentially natural.
Named Color Varieties
Bombay refers to pearls with a cream body color with a rose overtone.
Ceylon or Madras refers to white or cream pearls with fancy overtones of green, blue, or purple.
Names Based on Regions
La Paz pearls come from the coast off Baja California, Mexico.
Today, the term Oriental applies only to natural pearls from the Persian Gulf. In the past, the term referred to any pearls of saltwater origin.
Many gem materials have been called pearls due to their appearance and watery origins. Some of these materials are now referred to as calcareous concretions or non-nacreous pearls. They aren’t true pearls, despite their names. Some well-known examples of calcareous concretions include conch pearls and tridacna pearls.
So-called blister pearls develop attached to the inside of a mollusk’s shell. These dome-shaped, hollow pieces aren’t true pearls.
Some artisans create mabe pearls by filling blister pearls and gluing them to a shell backing.
Other “pearls” in name only include the following:
- French pearl is a misnomer for a piece of shell.
- Hinge pearls are the actual hinges of the shell.
- Osmenda pearls are cabochons cut from a chambered nautilus shell.
- Mother of Pearl is the iridescent inner layer found in most seashells.
Miscellaneous Trade Names
- Maiden pearls are newly harvested pearls.
- Dust pearls are too small for jewelry use. Usually, they measure less than 1 mm or 1/25 grains.
- Seed pearls are less than 2 mm or 1/4 grain. Usually, they have asymmetrical or off-round shapes.
- Half or ¾ pearls have flat backs and are usually drilled.
- Button pearls are round with flat backs.
Baroque pearls (top), Biwa pearls (middle), and teardrop pearls (pair). Biwa are a Japanese freshwater variety of pearl. Photo by Mauro Cateb. Licensed under CC By 2.0.
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 pearls 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. Pearls are very heat sensitive.
- Pearl rings and bracelets should have protective settings. If they don’t, wear them only occasionally rather than daily.
Mermaid and starfish earrings with 6 mm cultured pearls and blue sapphire accents. © CustomMade. Used with permission.