A Sea of History

STORY & PHOTOS BY RICHARD GROSS

Ancient coral reefs off the western coast of central and south Florida must have been beautiful when living. Now, agatized coral geodes bring that beauty to the present.

HOW AGATIZED CORAL FORMED

Florida agatized corals are the skeletal remains of colonial animals, known as anthozoans, that died 25 to 38 million years ago. The actual skeletal structures are very easy to see in some specimens and can be used to identify the coral species.

The skeletons consist of limestone (CaCO3 ) and the coral becomes agatized when the coral voids are filled with silica-rich water and the limestone is replaced by chalcedony quartz (SiO2 ) and common opal.

Chalcedony that is translucent and colored is called agate.

The replacement process takes millions of years and results in what geologists refer to as a pseudomorph –a crystal consisting of one mineral, but having the form of another which it has replaced. Occasionally, very small clear visible quartz crystals will form after the chalcedony layer; but, it is the richly colorful agate layer that we find so attractive.