Florida Geological Survey - Geology Topics
Identification: Agate, or chalcedony, is a
variety of cryptocrystalline quartz (SiO2). It
is found in a variety of colors, typically gray, brown, black, white, and
sometimes red. Fossil corals and mollusks may be replaced with agate
deposited by silica-rich ground water percolating through limestone. In 1979
the Florida Legislature designated agatized coral as the Florida State
Stone. It is described in the statute as “a chalcedony pseudomorph after
coral, appearing as limestone geodes lined with botryoidal agate or quartz
crystals and drusy quartz fingers, indigenous to Florida.”
Occurrence: Much of Florida’s agate, including the Tampa Bay
agatized coral, formed in the Oligocene-Miocene Hawthorn Group sediments
(see time scale). Once abundant at Ballast Point
in Tampa, it is occasionally dredged up in the Tampa and Clearwater areas.
It also occurs in limestones along the Econfina, Withlachoochee and Suwannee
Rivers. An Oligocene variety is sometimes found in Suwannee Limestone
quarries north of Tampa.
Use: Agatized coral, particularly in the form of large geodes,
is prized by gem and mineral collectors.