Calcareous Concretion (Non-Nacreous Pearl): Types & Values

Calcareous concretions are rare gemstones similar to pearls but with a porcelain-like appearance instead of the characteristic nacre that gives pearls their luster.

Despite lacking nacre, calcareous concretions are still gorgeous with unique patterns you won’t see in pearls.

That said, many gemologists classify pearls as concretions and call calcareous concretions “non-nacreous pearls.”

Examples of calcareous concretion gemstones we’ll be discussing are:

  • Conch Pearls

  • Tridacna Pearls

  • Melo Pearls

  • Quahog Pearls

  • Scallop Pearls

Today, our experts at Gem Rock Auctions will be discussing each calcareous concretion’s characteristics, along with the history, prices, and properties of calcareous concretions.

calcareous concretion non-nacreous pearl gemstonePictured above: Replica of the largest non-nacreous pearl ever found in a Tridacna Gigas (giant clam) named "Pearl of Allah" | Image credit: Hannes Grobe/AWI, CC-BY-SA-3.0

What Are Calcareous Concretions?

In gemology, calcareous concretions are pearl-like semi-precious gemstones, similarly found in various colors.

As pearl-like gems, calcareous concretions are June birthstones, astrologically lucky for Gemini & Cancer, and 30th wedding anniversary gems.

The definition of calcareous concretions in geology differs.

Geological concretions are cemented bodies of sediment found in sedimentary rocks. They can technically be a mineral or rock habit.

Another geological definition: mineral matter masses formed around a nucleus like a fossil or pebble. These concretions can look round, oblong, irregular, or disk-shaped. Concretions around fossils are popular.

When the mineral matter is calcium carbonate, it’s a calcareous concretion.

What’s the difference between a geode and a concretion? Both are rounded masses containing minerals, but geodes are hollow in the center and don’t form around a nucleus.

Non-Nacreous Pearl vs Calcareous Concretion

Stances diverse on whether all pearls must have nacre.

The International Gem Society (IGS) requires pearls to have nacre, opting to call non-nacreous pearls “calcareous concretions.”

However, the Gemological Institute of America (GIA) and World Jewellery Confederation (CIBJO) prefer “non-nacreous pearl” over “calcareous concretion.”

The US Federal Trade Commission (FTC) doesn’t require sellers to distinguish the two.

Calcareous Concretions Specifications & Characteristics

All calcareous concretion gems are classified as mineraloids and organic gemstones. Their composition is mostly calcium carbonate (aragonite, calcite, or both), water, and conchiolin (an organic protein).

The calcite/aragonite layers in calcareous concretions are coarser than those in nacreous pearls. Calcareous concretions don’t have nacre because a) they lack the layered aragonite platelet microstructure of pearls, and b) the inner shells of the mollusks forming them don’t have nacre.

Additionally, nacreous pearls have more aragonite than calcite, while non-nacreous pearls are vice versa. Aragonite has greater luster than calcite.

Calcareous concretion properties listed, separated by type when applicable:

  • Mohs hardness: Conch & Tridacna - 2.5 to 4; Melo - 5; Quahog - 2.5 to 4.5

  • Color: Conch - Pink, orange, yellow, white, brown; Tridacna - White, off-white, yellow; Melo - Shades of orange, yellow-orange, pale yellow, off-white, tan, brown; Quahog - Shades of purple, white, brown; Scallop -

  • Crystal structure: None (amorphous)

  • Luster: Conch & Tridacna - Sub-vitreous to dull; Melo & Quahog - Vitreous to dull

  • Transparency: Conch & Tridacna - Opaque; Melo & Quahog - Translucent to opaque

  • Refractive index: Conch, Tridacna & Quahog - 1.530 to 1.685; Melo - 1.54 to 1.64

  • Density: Conch & Tridacna - 2.18 to 2.89; Melo - 2.83 to 2.87; Quahog - 2.61 to 2.85

  • Cleavage: None

  • Fracture: Uneven

  • Streak: White

  • Luminescence: Variable fluorescence

  • Pleochroism: None

  • Birefringence: Conch & Tridacna - None; Melo & Quahog - 0.155

  • Dispersion: None

  • Optical effects: Conch, Tridacna & Melo - Chatoyant-like flame structure

Types of Calcareous Concretions

The calcareous concretion varieties most seen on the gem market are:


pink conch pearl calcareous concretion varietyPictured above: Natural, 1.95-carat pink conch pearl with flame structure | Image credit: Global Gemology, CC-BY-SA-4.0

Conch pearls come from the queen conch gastropod, Strombus gigas. A common misnomer is “Nassau pearls.” They’re best-known in pink but come in other hues.

Thanks to the mostly aragonite microcrystalline fiber bundles being aligned in a criss-cross pattern, conch pearls have a “flame structure” — a somewhat iridescent, chatoyant-like sheen resembling rolling flames or moire silk.

Conch pearls are cited among the rarest types of pearls, largely due to over-harvesting. There’s only one in every 10,000-20,000 shells and just 10 percent are gem-quality.


tridacna pearl of truth calcareous concretion varietyPictured above: The  39,925-carat "Pearl of Truth" pearl from a Tridacna Gigas | Image credit: Tagdance, CC-BY-SA-4.0

Tridacna pearls come from the Tridacna genus of giant saltwater clams, mostly the Tridacna gigas. The pearls are also called kima, from Indonesian. A common misnomer is “coconut pearls.”

These calcareous concretions are known for their potentially huge size, bright colors, and flame structure. The latter is similar to that of conch pearls, but the aragonite arrangement is radial, not in sheets.


papaya orange melo pearl ring with diamondsPictured above:  A melo pearl, diamond, and platinum ring. (Photo by Scott Papper, GG, CMA, RGA, AJP) | Image credit: Global Gemology, CC-BY-SA-4.0

Melo pearls are extremely rare — often cited as the rarest pearls — and come from gastropod sea snails called Melo Melo, AKA “bailer shell” or “Indian volute.”

Like Conch & Tridacna, Melo pearls have a flame structure. They’re found in shades of orange or brown. Vibrant “papaya” orange is the rarest and most sought-after.


quahog pearls - calcareous concretion varietyPictured above: A collection of natural quahog pearls | Image credit: Global Gemology, CC-BY-SA-4.0

Quahog (pronounced KO-hog) pearls come from the bivalve saltwater clam Mercenaria mercenaria, formerly Venus Mercenaria. They can be white to brown, but are best known for their purple hues.

Natural quahog pearls are among the rarest; most harvesters find one for every 5,000 Quahog clams opened, and many aren’t gem-quality. Plus, harvesting often kills the clam or damages the pearls.

Most Quahog pearls are oval. Color zoning is common. Some Quahog pearls have a circle of lighter color called an “eye effect” that makes the gem look translucent.


scallop pearl gemstone - calcareous concretion varietyPictured above: Scallop pearl | Image credit: David LeBlanc, CC-BY-SA-4.0

Scallop pearls — aka “Lion’s Paw” or “Mano de Leon” pearls — come from four saltwater scallops:

  1. Atlantic Lion’s Paw (Nodipecten nodosus)

  2. Pacific Lion’s Paw (Nodipecten subnodosus)

  3. Atlantic Sea scallop (Placopecten magellanicus)

  4. Atlantic Bay scallop (Argopecten irradians)

These calcareous concretions come in oval, button, and often baroque shapes. Their colors also vary, from shades of white, black, salmon, or brown to the rarer pink, purple, or burgundy hues.

Scallop pearls are unique in having patchwork or mosaic patterns, fibrous calcite cells in a honeycomb structure, and sometimes near-metallic luster.

They’re quite rare, at one pearl for every 10,000 scallops. Only around 20 percent are gem-quality.

Scallop pearls have gained popularity over the last 30 years.

1886 illustration of pink calcareous concretion pearl in conch shellPictured above: Illustration of pink pearl on conch shell entitled Pearls and Pearling Life, by Edwin W. Streeter in 1886 | Image credit: Freshwater and Marine Image Bank, Public domain

Calcareous Concretions Meaning & History

The meaning and healing properties of calcareous concretions are similar to nacreous pearls — read our Pearl Info Guide for more.

“Concretion” comes from the Latin concretio, meaning “compacting or condensing solid matter.”

“Calcareous” comes from the Latin calcārius, meaning “of or pertaining to lime” — lime is an old term for a calcium-rich material.

Each calcareous concretion variety has its own unique historical side:

  • Conch Pearls: Popular among Native Americans for centuries, likely brought to Europe by Christopher Columbus in 1500s; Classified in 1758

  • Tridacna Pearls: Famously first fossil pearls documented in 1723

  • Melo Pearls: Historically symbolized Eastern royalty, worshiped as sacred, and thought to have originated when dropped from a flying dragon

  • Quahog Pearls: Once used as currency among New England Native Americans

The popularity of scallop pearls took off in the late 1990s, when natural pearl demand took over the previous 80-year cultured pearl popularity. Nova Scotia jeweler Craig Fancy pioneered scallop pearl jewelry.

conch pearl calcareous concretion brooch from 1900Pictured above: Marcus and Co. Brooch from ca. 1900 made of plique-à-jour enamel, conch pearl, diamond, platinum and eighteen karat gold | Image credit: Metropolitan Museum of Art, Public domain

Calcareous Concretions Gemstone Properties

Calcareous concretions are valuable, but their specific value depends on factors similar to pearl grading.


Overall, calcareous concretions with more saturated and vibrant colors carry greater value.

But each calcareous concretion type has more valuable hues:

  • Conch: Rarest & most valuable are pink or white; Somewhat valuable in orange or yellow; Least valuable & most common are brown

  • Tridacna: Best with even color distribution

  • Melo: Rarest & most valuable in vibrant papaya orange

  • Quahog: Most valuable in purple hues; Brown shades slightly more costly than white; “Eye effect” is sometimes valuable

  • Scallop: Shades of purple, pink, or maroon are rarer & more valuable; White or brown shades more common

Even color distribution is important for the value of Tridacna, Melo, and Quahog pearls unless they have symmetrical color zoning/bands.

Flame Structure / Pattern

Most calcareous concretions have the flame structure mentioned earlier. The effect may only be visible under magnification, but dramatic flame structure will greatly boost (sometimes doubling) value.

Inversely, a flame structure that isn’t clearly visible will significantly decrease value.

The other value-boosting look is the mosaic pattern in scallop pearls.


Calcareous concretions are best with a sub-vitreous luster resembling porcelain. Dull or low luster decreases value. Scallop pearls may be higher value with luster closer to metallic.

Surface Blemishing

The most valuable calcareous concretions have little to no surface blemishes or roughness, but all pearls have some imperfections.

Common imperfections on non-nacreous pearls are:

  • Dimples

  • Bumps

  • Pits

  • Cracks

Imperfections are more common in certain types — many Melo pearls have surface blemishes but imperfections aren’t usually noticeable on Quahog pearls.

More noticeable imperfections greatly lower value.

papaya orange melo pearl necklace with diamondsPictured above: A melo pearl, diamond and platinum necklace | Image credit: Global Gemology, CC-BY-SA-4.0

Shape & Symmetry

Calcareous concretions aren’t often round like nacreous pearls, making round non-nacreous pearls the rarest and highest-value.

Usually, the best ones have symmetrical oval shapes, followed by symmetrical teardrop and egg shapes. Value decreases for less symmetrical or ovoid shapes.

Other calcareous concretions shapes include button, drop, baroque, and irregular.


Different non-nacreous pearls differ in size ranges, but larger sizes (particularly of higher quality) can be vastly more valuable.

Size ranges for each calcareous concretion variety are:

  • Conch: Usually under 3 mm or 0.2-0.3 cts, can be up to 22 mm or 45 cts

  • Tridacna: Usually 3 to 140 mm; Known as largest, size doesn’t significant change price unless huge

  • Melo: Usually 7-11 mm, sometimes 30 mm; Can be over 200 mm long

  • Quahog: Usually 3-8 mm or below 10 cts; Up to 44 mm long

  • Scallop: Usually 2-11 mm, up to 40 cts

The three largest authenticated, natural, non-nacreous pearls are all Tridacna.

  • 2023 Find — 105.38 lbs

  • “Giga Pearl” — 15.5 in diameter, 60.95 lbs

  • “Pearl of Lao Tzu” (or “Pearl of Allah”) — 9.45 in diameter, 14.1 lbs

Natural vs Cultured

Most non-nacreous pearls can’t be cultured. One exception was developed by Florida scientists around 2007: cultured conch pearls, which hit markets in 2010.

tridacna gigas giant clam that makes calcareous concretions in spain museumPictured above: Tridacna gigas (Giant clam) in Aquarium Finisterrae in Spain | Image credit: Fernando Losada Rodríguez, CC-BY-SA-4.0

Calcareous Concretions Formation & Sources

How are concretions formed? Calcareous concretion formation is similar to nacreous pearl formation.

An irritant enters the mollusk, and the creature secretes layers of fibrous crystals and conchiolin around the irritant. The creatures behind calcareous concretions just have different ratios of materials for wrapping up the irritant.


The main (or only) sources for each calcareous concretion variety are:

  • Conch: Bahama Seas, Caribbean Seas & Florida Straits

  • Tridacna: Reefs around Indonesia and the Philippines

  • Melo: Only in Southeast Asia (Myanmar, Malaysia, Thailand, South China Sea, and the Philippines)

  • Quahog: Eastern North America & Central America

  • Scallop: Native to Baja California coast

hand wearing scallop pearl ringPictured above: Scallop pearl ring | Image credit: Nicole Bratt, Flickr, CC-BY-SA-2.0

Calcareous Concretions Price & Value

Based on quality, price varies broadly within each type.

The general calcareous concretion price ranges to expect:

  • Conch: $250 to $9,000 per carat

  • Tridacna: $50 to $250 per carat

  • Melo: $250 to $3,000 per carat

  • Quahog: $250 to $9,000 each

  • Scallop: $100 to $2,500 per carat

Significantly large or extremely rare examples can reach prices over $50,000.

Calcareous Concretions Care and Maintenance

While calcareous concretions can be slightly more durable than nacreous pearls, gemstone care is still crucial. The main tips are:

  1. Store out of direct sunlight — exposure can fade color — and separately from other gems.

  2. Consult an expert before polishing.

  3. Opt for protective settings on jewelry.

  4. Reserve jewelry for evening or occasional wear.

  5. Wear jewelry over clothes (not on skin).

  6. Keep away from heat.

Only clean these gems by gently wiping them with cloth dipped in a solution of water and mild soap. Wipe the gems down after each time you wear them.

purple quahog pearl ring with diamondsPictured above: A large purple quahog pearl, diamond, and platinum ring | Image credit: Global Gemology, CC-BY-SA-4.0

Enjoy the Unique Features of Calcareous Concretions!

The name of these gems doesn’t do them justice — calcareous concretions are gorgeous gems with unique patterns, origins, and looks. Add in the alluring rarity of these non-nacreous pearls and you have an unconventional gem worthy of seeking out!

Explore our array of nacreous and non-nacreous pearls today!

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