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Brooch with coral pieces
MFA# 43.2121


Skelton of coral colony

1) The Calcium carbonate structure composed of the residual skeletons from colonies of soft-bodied marine animals of the class Anthozoa (phylum Cnidaria). Coral deposits accumulate at an average rate of about 0.5 to 2.8 cm per year to form reefs or islands in warm seas (13-15 C). Coral species are separated into hard (with skeletons) and soft (without skeletons). The hard, or calcareous group, includes: stony corals (star coral), black corals, thorny corals, horny corals (gorgonians), and blue corals. They can range in color from white, cream, red, blue, and black. Red, or precious, coral is considered the most valuable. It is obtained from the Mediterranean Sea and off the coast of Japan. Black coral can be bleached to give a 'golden' appearance. Hard corals have been used since antiquity to make jewelry, ornaments, beads, and decorative objects. The soft group of corals are bendable and only contain colonies of polyps.

Florida corals

2) A pale orangy pink color.

Coral color pigment

Synonyms and Related Terms

calcium carbonate; coral (Esp., Port.); Koralle (Deut.); corail (Fr.); corallo (It.); sango matsu (Jap.); koraal (Ned.)

Physical and Chemical Properties

For calcareous corals:

  • Semitranslucent to opaque
  • Luster = waxy to vitreous
  • Fracture = conchoidal to splintery to uneven (depending on species)
  • Fluorescence = inert to orange under LW and SW
Mohs Hardness 3.5
Density 2.60-2.70 g/ml
Refractive index 1.486 - 1.658
Birefringence 0.172

Resources and Citations

  • Gem Identification Lab Manual, Gemological Institute of America, 2016.
  • The American Heritage Dictionary or Encarta, via Microsoft Bookshelf 98, Microsoft Corp., 1998
  • Encyclopedia Britannica, Comment: Coral. Retrieved May 26, 2003.
  • R. J. Gettens, G.L. Stout, Painting Materials, A Short Encyclopaedia, Dover Publications, New York, 1966
  • G.S.Brady, Materials Handbook, McGraw-Hill Book Co., New York, 1971 Comment: p. 235
  • Dictionary of Building Preservation, Ward Bucher, ed., John Wiley & Sons, Inc., New York City, 1996
  • Tom Rowland, Noel Riley, A-Z Guide to Cleaning, Conserving and Repairing Antiques, Constable and Co., Ltd., London, 1981
  • Oppi Untracht, Jewelry Concepts and Technology, Doubleday & Co., Inc., New York City, 1985
  • Jack Odgen, Jewellery of the Ancient World, Rizzoli International Publications Inc., New York City, 1982
  • Art and Architecture Thesaurus Online,, J. Paul Getty Trust, Los Angeles, 2000