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. 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.