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Agatized (silicified) coral, photo 1
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Named stone

Agatized (silicified) coral

Also called Tampa Bay coral, Florida agatized coral, Petrified coral, Silicified coral

A kind of chalcedony

Agatized (silicified) coralChalcedonySoutheast

3 photos

Description

Agatized coral is Florida's showy toolstone: fossil coral whose original skeleton was replaced by waxy, translucent chalcedony, often botryoidal, in blue, red, brown, amber, white or black. It occurs as geodes where replacement was partial and as solid pieces where it was total. It was first found around Tampa Bay in the 1820s and also turns up near Tarpon Springs, south of New Port Richey, near Kathleen and along the Suwannee River. In the Tampa Bay area the Tampa Member of the Arcadia Formation holds corals as silicified pseudomorphs. Mississippi's geologists call it intensely utilized, and compare Tallahatta agate to it.

How to identify it

Colour
Blue, red, brown, amber, white, black or a combination (USGS).
Texture
Waxy chalcedony; geodes are partial replacements with a botryoidal lining, solid pieces are complete replacements.
Luster
Waxy, translucent.
Inclusions
Coral structure preserved in the replaced stone.

Geology

Fossil coral replaced by silica. The Tampa Member of the Arcadia Formation (Oligocene to Miocene) holds mollusks and corals as molds, casts and silicified pseudomorphs. The USGS says the coral is replaced by waxy, translucent, botryoidal, varicolored chalcedony, as geodes (partial replacement) and as solid pieces (total replacement).

Where it comes from

Southeast

States

Quarries and outcrops

  • Tampa Bay area (first found about 1825)
  • Tarpon Springs, Pasco County area south of New Port Richey, and Kathleen
  • Suwannee River banks in Hamilton, Columbia and Suwannee counties

Used by

  • Early peoples made tools and weapons from chalcedony varieties including agatized coral (USGS)
  • Described by Mississippi DEQ as 'intensely utilized' in Florida

Look-alikes

Stones that are easy to confuse with Agatized (silicified) coral.

  • Tallahatta agate: remarkably similar, but its laminar bedding often spoils knapping (MDEQ)
  • Chert from Florida limestone

Sources

  1. U.S. Geological Survey. 1995. Gemstones: Chalcedony (SP-14-95), Mineral Information archive.apps.usgs.gov (opens in a new tab)
  2. Mississippi Department of Environmental Quality, Office of Geology. Lithic Materials (web page; updated December 2023).mdeq.ms.gov (opens in a new tab)
  3. Scott, Thomas M. 2001. Text to Accompany the Geologic Map of Florida. Florida Geological Survey Open-File Report 80.ufdcimages.uflib.ufl.edu (opens in a new tab)

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