Calcareous Concretions | Gemopedia

All pearls are, by definition, calcareous concretions because their natural nacre is calcium-based. However, some "pearls," which are defined as nacreous calcareous concretions, are not technically pearls. Two prime examples are pearls from the conch and the melo melo snail. These gems are considered a pearl within the trade, but not by definition as they are not made by nacreous coatings. Their creation is almost identical to how true pearls form, with the exception that the calcareous concretion that makes the gem is not true nacre.
Colors
Pink, Red, Light To Dark Purple, White, Brown

Calcareous Concretions Classification

Common Name

Calcareous Concretions

Species

NA

Variety

Colors

Pink, Red, Light To Dark Purple, White, Brown

Alternate Names

Gemstone Groups

Key Separations

RI, birefringence blink, magnification and appearance.

Comments

Use birefringence blink method to obtain birefringence. Brown conch "pearls" have an SG 2.18 to 2.77. "Pearls" from the Pinna shell mollusk, the common clam (venus mercenaria), ostrea virinica and ostrea edulis. Conch and Melo pearls are not considered true pearls.

Calcareous Concretions Optical Properties

Transparency

Translucent - Opaque

Refractive Index

1.53-1.685

Birefringence

0.155

Optic Character

Optic Sign

Polariscope Reaction

Aggregate (AGG)

Fluorescence

SWUV: Variable
LWUV: Variable

CCF Reaction

Pleochroism

None

Dispersion

Comments

Calcareous Concretions Chemistry & Crystallography

Chemical Name

calcium carbonate, conchiolin and water

Chemical Formula

CaCO3 with H2O and other organic materials

Synthesis

Crystal System

NA

Classification

Carbonate

Nature

Natural

Crystallinity

Aggregate

Comments

Calcareous Concretions Characteristic Physical Properties

Hardness

2.5-4

Streak

Specific Gravity

2.83-2.87 Typical:2.85

Toughness

Fair

Inclusions

Luster

Dull

Stability

Poor

Fracture

Uneven, Splintery

Cleavage

None

Comments

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