Calcareous Concretions Gemstone & Information | Gemopedia by JTV | 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.
Pink, Red, Light To Dark Purple, White, Brown
Calcareous-concretions classificationCommonNameCalcareous ConcretionsSpeciesNAVarietyColorsPink, Red, Light To Dark Purple, White, BrownAlternateNamesGemstoneGroupsKeySeparationsRI, birefringence blink, magnification and appearance.ClassificationCommentsUse 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 & crystallographyChemicalNamecalcium carbonate, conchiolin and waterChemicalFormulaCaCO3 with H2O and other organic materialsSynthesisCrystalSystemNAChemistryClassificationCarbonateNatureNaturalCrystallinityAggregateChemistryComments
Calcareous-concretions optical propertiesTransparencyTranslucent - OpaqueDispersionOpticalCommentsRefractiveIndex1.53-1.685Birefringence0.155OpticCharacterOpticSignPolariscopeReactionAggregate (AGG)FluorescenceSWUV: Variable
Calcareous-concretions characteristic physical propertiesHardness2.5-4CharacteristicCommentsStreakSpecificGravity2.83-2.87 Typical:2.85ToughnessFairInclusionsLusterDullStabilityPoorFractureUneven, SplinteryCleavageNone