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Fossil Coral

By Tim Matthews, JD, FGA, GG, DGA,
CEO and President of JTV (retired)
Published: June 2014
Modified: September 2023
Fossil Coral Polished Fossil Coral Rough Fossil Coral Jewelry
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Table of Contents
  • General Information
  • Fossil Coral Colors
  • Alternate Names
  • Countries of Origin
  • History
  • Care

Coral is the calcareous skeleton of marine animals known as coral polyps. The oldest known corals date back to 500 million years ago. In fossil coral the aragonite of the original structure is replaced by calcite or agate. The fossilization process preserves the ancient corals and makes very attractive cabochons that can be used in jewelry. Most fossilized coral in jewelry has been replaced with agate.

General Information

  • Classification
  • Optical Properties
  • Characteristic Physical Properties
  • Chemistry & Crystallography
Common Name
Fossil Coral
Transparency
Translucent-Semitransparent
Dispersion
Strength: None
Refractive Index
1.535-1.539
Birefringence
0.004
Optic Character
NA
Optic Sign
NA
Polariscope Reaction
Aggregate (AGG)
Fluorescence
SWUV: Inert
LWUV: Inert
Pleochroism
None
Hardness
6.5-7
Specific Gravity
2.550-2.700 Typical:2.600
Toughness
Excellent
Inclusions
Fossil coral will show the original structure of the coral.
Luster
Greasy, Vitreous
Stability
Good
Fracture
Conchoidal, Granular
Cleavage
None
Chemical Name
silicon dioxide (aka silica)
Chemical Formula
SiO2
Crystal System
Trigonal
Chemistry Classification
Silicate

Fossil Coral Colors

  • Bi-color Fossil Coral
    Bi-color
  • Black Fossil Coral
    Black
  • Brown Fossil Coral
    Brown
  • Colorless Fossil Coral
    Colorless
  • Gray Fossil Coral
    Gray
  • Multi-color Fossil Coral
    Multi-color
  • White Fossil Coral
    White
  • Yellow Fossil Coral
    Yellow

Alternate Names

agatized coral, petrified coral

Countries of Origin

Russian Federation; Morocco; China; United States of America; United Kingdom of Great Britain and Northern Ireland; Germany; Indonesia

History

The oldest coral fossils date back to approximately 500 million years ago. There have been several prehistoric coral reef extinction events due to changes in climate or geological events. Most of the fossilized coral used in jewelry today comes from Indonesia. The Indonesian fossilized coral is approximately 20 million years old. It is believed that volcanic eruptions in the area buried the coral, and the volcanic ash provided the silica for the agate replacement of the original calcium carbonate skeleton. The ash also contained the iron and manganese that gives the material its vivid colors. Petoskey stones are the state stone of Michigan and can be found in Michigan rock quarries and on the shores of Lake Michigan and Lake Huron. They are type of rugose coral that lived in the area 350 million years ago. Agatized coral is the state gemstone of Florida. It is 25 to 38million years old and can be found in the Tampa Bay area and the Withlacoochee, Suwannee, and Econfina River valleys.

Care

Normal, gentle care care.

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