Florida Geological Survey -
Geology Topics
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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.
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