Skip to main content
Geosciences LibreTexts

16.29: Coral

  • Page ID
    4365
  • Coral
    Chemical composition Aragonite (CaCO3)or conchioline
    Crystal system Amorphous
    Habit Branching, tree-like
    Hardness 2.5 - 3.5
    Refractive index 1.48 - 1.56
    Specific gravity 1.37 - 2.60
    Lustre Waxy

    File:Coral beads.jpg

    Figure \(\PageIndex{1}\): Mediterranean Red Coral Beads

    File:Red coral.jpg

    Figure \(\PageIndex{2}\): Coral, Species: Scleronephthya


    Coral is one of the few organic gems. It is the exoskeleton of colonial polyps. These are small animals that create their branching homes from calcium carbonate dissolved in seawater. The colors of gem coral range from pale pink (angel-skin) to oranges and reds. Coral has been used throughout our history; it was pictured on a Sumerian vase dated at 3000 BC It was traded between the Mediterranean and India at the time of the birth of Christ. Coral was introduced into Mesoamerica by the Spaniards. Native Americans used it extensively in jewelry. Coral was used in Roman times to protect children from danger. It is still worn in Italy today to protect from the evil eye.