Several species of marine mollusks produce stony growths called calcareous concretions. The most commonly encountered in jewelry are conch pearls from the Queen conch, tridacna pearls from the giant clam, and melo pearls from the bailer shell snail. Scallop pearls are relatively recent arrivals on the market. Chemically, calcareous concretions are closely related to pearls, consisting primarily of aragonite, calcite, conchiolin, and water. However, these growths are coarser and lack nacre. Nevertheless, many calcareous concretions have other desirable qualities for jewelry enthusiasts and gem collectors.
Calcareous Concretions Value
Calcareous concretions are valued much like pearls. Value goes to the brighter colors and stronger saturations. Shape values are based on symmetry, if not round or oval. Smoother surfaces, higher luster, and larger sizes also increase value.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
Calcareous Concretions Information
|Refractive Index||Conch and tridacna pearls: 1.53 to 1.685. Melo Pearls: 1.54 to 1.64.|
|Hardness||Conch and tridacna pearls: 2.5 to 4 Melo Pearls: 5.|
|Specific Gravity||Conch and tridacna pearls: 2.18 to 2.89.|
|Occurrence||Formed within various marine mollusks. See "Comments" below.|
People have used “pearl” names for calcareous concretions for centuries. The terms are still used today. Both the World Jewellery Confederation (CIBJO) and Gemological Institute of America (GIA) refer to calcareous concretions as non-nacreous pearls. The International Gem Society (IGS) prefers the term calcareous concretions but acknowledges popular usage.
Occasionally, pen shell mollusks (genus Pinna) may produce pink or red concretions. Edible oysters (such as Ostrea edulis and Crassostrea virginica) may produce white or purple stones. Many members of the clam family (Bivalvia) can produce calcareous concretions of white, brown, and light to dark purple. However, consumers rarely consider then attractive enough to value as gems.
Conch pearls (pronounced Conk) lack an iridescent nacre layer. However, they are rare and expensive organic gems. Prices can run as high as $9,000 per carat. The Queen conch (Strombus gigas) is found primarily in the Florida Straits and the Bahamian and Caribbean Seas. Once common, their popularity as a culinary delicacy caused excessive harvesting.
In the late 1930s, La Place Bostwick very likely successfully cultivated conch pearls. However, his techniques remained a mystery. In 2009, Dr. Hector Acosta-Salmon and Dr. Megan Davis developed a reliable technique for conch pearl cultivation.
Giant clams (genus Tridacna) can produce large, white calcareous concretions called tridacna pearls. They live in the Pacific and Indian Oceans.
Bailer shell snails (Melo melo), also called Indian volute snails, live in the waters off South East Asia (roughly from Myanmar to Indonesia). Although predators, these rare snails are considered delicacies themselves. Unfortunately, cooking damages any calcareous concretions, melo pearls, that may rest inside the hapless mollusks. In Myanmar, the shell is called a “coconut shell” (ohn kayu) and the gems “coconut pearls” (ohn pale).
Marine scallops native to the coasts of Baja California produce the calcareous concretions called scallop pearls. Their shapes range from round to highly irregular.
Conch, tridacna, and melo pearls have a distinctive, flame-like pattern. Scallop pearls have a mosaic pattern. Imitations haven’t duplicated these patterns, so their identification is very simple and straight forward.
Because of their similar patterns, color best distinguishes conch and melo pearls. While conch pearls are primarily pink, melo pearls are usually orange. Their top color is an intense orange described as “papaya.” Melo pearl colors range from near white to pale yellow through yellowish orange, orange, and brownish orange, with “porcelain-like” luster.
Melo pearl shapes range from baroque through oval to spherical.
Melo pearls are not light stable. Their colors will fade when exposed to ultraviolet light, including what comes from the sun. Pale colors likely signify the gems have been displayed outside for some time.
In addition to their mosaic pattern, scallop pearls have a unique appearance. Colors vary from cream, salmon, and mauve, with distinct brown tones, an almost metallic luster, and a chatoyant sheen.
Calcareous concretions may grow very large. Melo pearls can reach over 200 millimeters in length. Scallop pearls may weigh up to 40 carats.
The largest known calcareous concretion is the famous “Pearl of Lao Tzu,” also known as the “Pearl of Allah.” Found in the seas off the island of Palawan in the Philippines, this tridacna pearl weighs 14.1 pounds and measures 9.45 inches in diameter. Wilburn Dowell Cobb, the American who acquired the pearl in 1939, wrote an incredible (if romanticized) account of its discovery. (For the record, giant clams have never been known to drown humans).
- Nassau Pearls: misnomer for conch pearls.
- Coconut Pearls: misnomer for tridacna pearls.
The color fade of melo pearls is gradual. Don’t worry too much about brief exposures. However, reserve melo pearl jewelry for evening wear and special occasions. What these gems lose under sunlight they make up in hardness. At a 5, melo pearls are the hardest of all pearls and calcareous concretions.