Calcareous Concretions Gemstone & Information | Gemopedia by JTV | Gemopedia™

Calcareous Concretions

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

    CommonName
    Calcareous Concretions
    Species
    NA
    Variety
    Colors
    Pink, Red, Light To Dark Purple, White, Brown
    AlternateNames
    GemstoneGroups
    KeySeparations
    RI, birefringence blink, magnification and appearance.
    ClassificationComments
    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 chemistry & crystallography

    ChemicalName
    calcium carbonate, conchiolin and water
    ChemicalFormula
    CaCO3 with H2O and other organic materials
    Synthesis
    CrystalSystem
    NA
    ChemistryClassification
    Carbonate
    Nature
    Natural
    Crystallinity
    Aggregate
    ChemistryComments
  • Calcareous-concretions optical properties

    Transparency
    Translucent - Opaque
    Dispersion
    OpticalComments
    RefractiveIndex
    1.53-1.685
    Birefringence
    0.155
    OpticCharacter
    OpticSign
    PolariscopeReaction
    Aggregate (AGG)
    Fluorescence
    SWUV: Variable
    LWUV: Variable
    CCFReaction
    Pleochroism
    None
  • Calcareous-concretions characteristic physical properties

    Hardness
    2.5-4
    CharacteristicComments
    Streak
    SpecificGravity
    2.83-2.87 Typical:2.85
    Toughness
    Fair
    Inclusions
    Luster
    Dull
    Stability
    Poor
    Fracture
    Uneven, Splintery
    Cleavage
    None