Wooster’s Fossil of the Week: An agate-replaced coral from the Oligocene-Miocene of Florida

I long thought of this beautiful specimen as more rock than fossil. It is a scleractinian coral that has had its outer skeleton replaced by the silicate material agate and its interior skeleton completely hollowed out. The result is a geode that happens to also be a fossil.
Then during last month’s North American Paleontological Convention in Gainesville, Florida, I saw the above specimens on display in the Florida Museum of Natural History. These fossils were so striking that I decided to highlight our single example.
This is a view of the top surface of the Wooster specimen. In the upper left is an array of holes with crystals radiating away from them. These are remnants of the original corallites, and there is just enough information there for us to conclude the likely genus is Montastraea. This piece thus becomes an example of Florida’s official state stone. Here’s the official definition: “… a chalcedony pseudomorph after coral, appearing as limestone geodes lined with botryoidal agate or quartz crystals and drusy quartz fingers, indigenous to Florida.” Our specimen came from the Hawthorn Group of rocks near Tampa, Florida.
The outside of the fossil shows horizontal banding remaining from the original growth lines in the coral, which is another clue that this is Montastraea. The coral made its skeleton of aragonite around 30 million years ago. After death and burial, silica-rich groundwater began to replace the aragonite on the surface of the coral with what later became banded agate. The interior dissolved away into a hollow cavity.

The common name for this fossil is “agatized coral“, and it is a collector’s item. It is apparently Florida’s only native gemstone. Pretty cool that their state rock and gemstone is a fossil!

References:

Scott, T.M. 1990. The lithostratigraphy of the Hawthorn Group of peninsular Florida. World Phosphate Deposits 3: 325-336.