Calcareous Concretions Value, Price, and Jewelry Information

14k yellow gold ring with an oval, mottled pink conch pearl, 10 x 9.2 mm, in a textured, wirework cage setting. Photo courtesy of liveauctioneers.com and Hindman.

Calcareous Concretions

Several species of marine mollusks produce stony growths called calcareous concretions or non-nacreous pearls. Varieties such as conch pearls, tridacna pearls, and others are frequently used in jewelry. Although not true pearls, they can still make beautiful gems.

Calcareous Concretions Value

Generally, calcareous concretions are valued much like pearls. Brighter colors and stronger saturations command higher prices. Rounds and ovals are more desirable, and other shapes are judged based on how symmetrical they appear. Smoother surfaces, higher luster, and larger sizes also increase value.

Calcareous concretions can also have striking features not seen in true pearls. To learn more about quality factors for specific calcareous concretions, consult our buying guides for conch pearls, tridacna pearls, melo pearls, and quahog pearls.

melo pearl ring

Melo pearls, like the beautiful, round, 54.45-ct specimen in this gold ring, can show a flame-like effect on their surface. Photo courtesy of liveauctioneers.com and Casa d’aste ARCADIA.

The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.

Pearls via Gem Price Guide

Prices of pearls are based on several quality factors. The values provided in this guide are for round cultured pearls. Baroque pearls are about 25-35% the cost of round pearls. Natural pearls are extremely rare, and largely limited to auction and collector’s markets. These can be worth 10 to 20 times an equivalent Akoya cultured pearl.
Akoya Pearls with White Body Color
Matching Pair
Size (mm) Top Luster High Luster
6 - 6.5 - -
6.5 - 7 - -
7 - 7.5 - -
7.5 - 8 - -
8 - 8.5 - -
8.5 - 9 - -
9 - 9.5 - -
Single Pearl
Size (mm) Top Luster High Luster
6 - 6.5 - -
6.5 - 7 - -
7 - 7.50 - -
7.5 - 8 - -
8 - 8.5 - -
8.5 - 9 - -
9 - 9.5 - -
Freshwater Pearls with White Body Color
Matching Pair
Size (mm) Top Luster High Luster
6 - 7 - -
7 - 8 - -
8 - 9 - -
9 - 10 - -
10 - 11 - -
Single Pearl
Size (mm) Top Luster High Luster
6 - 7 - -
7 - 8 - -
8 - 9 - -
9 - 10 - -
10 - 11 - -
South Sea Pearls
Matching Pair
Size (mm) High Luster, White High Luster, Golden
8 - 9 - -
9 - 10 - -
10 - 11 - -
11 - 12 - -
12 - 13 - -
13 - 14 - -
Single Pearl
Size (mm) High Luster, White High Luster, Golden
8 - 9 - -
9 - 10 - -
10 - 11 - -
11 - 12 - -
12 - 13 - -
13 - 14 - -
Tahitian Pearls with Black Body Color
Matching Pair
Size (mm) Top Luster High Luster
8 - 9 - -
9 - 10 - -
10 - 11 - -
11 - 12 - -
12 - 13 - -
Single Pearl
Size (mm) Top Luster High Luster
8 - 9 - -
9 - 10 - -
10 - 11 - -
11 - 12 - -
12 - 13 - -

Calcareous Concretions / Non-Nacreous Pearls Value via Gem Price Guide

Conch Pearls
All sizes
to ,000/ct
Tridacna Pearls
All Sizes
- / ct
Melo Pearls
All Sizes
- / ct

Calcareous Concretions Information

Data Value
Name Calcareous Concretions
Colors Red, pink, white, brown, light to dark purple; Melo Pearls: orange, white, pale yellow, yellowish orange, brownish orange
Crystallography Amorphous
Refractive Index Conch and tridacna pearls: 1.53 to 1.685. Melo Pearls: 1.54 to 1.64.
Hardness Conch and tridacna pearls: 2.5-4; Melo Pearls: 5.
Wearability Good to Poor
Fracture Uneven
Specific Gravity Conch and tridacna pearls: 2.18 to 2.89.
Cleavage None
Transparency Opaque
Birthstone June (pearls are the traditional birthstone)
Occurrence

Formed within various marine mollusks. See “Comments” below.

Tiffany and Company conch pearl pendant

Tiffany and Company platinum sautoir with diamonds and conch pearl pendant, on display at the Walters Art Museum. Public Domain. (Cropped to show detail).

Comments

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.

The calcareous concretions most commonly encountered in jewelry are conch pearls from the queen conch (Strombus gigas), tridacna pearls from the giant clam (genus Tridacna), melo pearls from the bailer shell snail (Melo melo), and quahog pearls from quahog clams (Mercenaria mercenaria). Scallop pearls are relatively recent arrivals on the market.

Tridacna gigas, the Giant Clam

Tridacna gigas, the giant clam. Photo by Brian Gatwicke. Licensed under CC By 2.0.

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 them attractive enough to value as gems.

What’s the Difference Between Calcareous Concretions and True Pearls?

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.

non-nacreous pearl and diamond ring

While the non-nacreous pearl in this three-stone diamond ring has a round shape like a classic true pearl, it also shows a faint, flame-like structure on its surface. This effect is due to the arrangement of aragonite in non-nacreous pearls and isn’t found in true, nacre-bearing pearls. Photo courtesy of liveauctioneers.com and Fellows.

Conch Pearls

Conch pearls (pronounced conk) lack an iridescent nacre layer. However, they’re 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, but his techniques remained a mystery. In 2009, Dr. Hector Acosta-Salmon and Dr. Megan Davis developed a reliable technique for conch pearl cultivation.

calcareous concretions - conch pearls

Tridacna Pearls

Giant clams (genus Tridacna) can produce large, white calcareous concretions called tridacna pearls. They live in the Pacific and Indian Oceans.

tridacna pearl necklace

Tridacna pearl necklace, 38 round beads, 104 g. Photo courtesy of liveauctioneers.com and Tai Ji International Auctions.

Melo Pearls

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 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).

calcareous concretions - melo pearl colors

Melo Pearls

Quahog Pearls

Mercenaria mercenaria clams off the coast of New England produce quahog pearls. These calcareous concretions can grow in a wide variety of shapes, in colors ranging from white to brown, lilac, and deep purple. Some may show color zoning.

purple quahog pearl

Natural purple quahog pearl, 6.07 cts, slightly oval-shaped, 6.07 x 5.64 mm. Photo courtesy of liveauctioneers.com and Kaminski Auctions.

Scallop Pearls

Marine scallops native to the coasts of Baja California produce the calcareous concretions called scallop pearls. Their shapes range from round to highly irregular.

calcareous concretions - scallop pearls

Scallop Pearls

Identifying Characteristics

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 melo shell and pearls

Melo melo shell and pearls at the Exposition Temporaire: Perles, National Museum of Natural History, Paris, France. Photo by Liné1. Licensed under CC By-SA 4.0.

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 - scallop pearl ring

Scallop pearl ring. Photo by Nicole Bratt. Licensed under CC By-SA 2.0.

Sizes

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).

Learn more about the odd and ongoing saga of this huge gem in our article on seven famous pearls.

calcareous concretions - Pearl of Lao Tau replica

A replica of the “Pearl of Lao Tzu,” on display in a shop in Tenerife, Canary Islands, Spain. Photo by hgrobe. Licensed under CC By 3.0.

Trade Names

  • Nassau Pearls: misnomer for conch pearls.
  • Coconut Pearls: misnomer for tridacna pearls.

Care

All pearls, nacreous or not, require special attention. See the main pearl listing as well as our detailed pearl care guide.

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, nacreous or not.

pink conch pearl ring - Mikimoto

18k gold ring with an oval-shaped, pink conch pearl and pear and marquise-cut diamonds. Mikimoto maker’s mark. Photo courtesy of liveauctioneers.com and Heritage Auctions.