Agatized Coral from Florida
State Marine Fossil • Botryoidal Interior
Collecting Locations & Age: Designated the official stone of Florida in 1979, these fossils are roughly 38-25 million years old and come from the Oligocene-Miocene period. Agatized coral is found in three main Florida locations: Tampa Bay, the Econfina River, and the Withlacoochee/Suwannee river beds and It is no longer permissible to collect specimens on public lands so they are getting harder and harder to find.
Preservation & Appearance: Coral is the outside skeleton of tiny ocean animals called polyps. These marine invertebrates live in colonies that are attached to hard underwater surfaces like rocks and sunken ships. When they are alive, polyps produce carbon dioxide which combines with the lime in warm seawater to form a limestone-like surface that is called coral. They basically build their own houses.
Agatized corals are found in ancient ocean beds, where silica rich groundwater has percolated through them and over time. When this happens it replaces the calcium carbonate skeleton with a hard variety of quartz called chalcedony. This long process (taking upward of 20-30 million years) results in the formation of a “pseudomorph,” meaning that one mineral has replaced another without having lost its original form. The by-product is an ornamental fossil with a sometimes banded stone look.
Corals are found in a variety of colors- white, gray, brown, black, yellow and red and it basically comes down to what minerals were present in the water when the corals were formed. Quite often you will find grape-like bumps inside the coral where the chalcedony has replaced the coral. The term for these “grapes” in the mineral world is botryoidal.
Uses: On the hobby side coral is great for cutting and polishing to display. Some folks like to make fossil coral jewelry.
From an educational standpoint this is great example of a pseudomorph fossil and a nice complement to other marine related items. In either case, you are welcome to request a PDF that provides you with additional information and photographs about this beautiful rock at no extra cost, just give us an electronic poke in the ribs when you check out.
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