Quick answer: Agatized coral is fossil coral whose original calcium carbonate skeleton was replaced by chalcedony, preserving the coral’s flower-like pattern in quartz. Florida’s examples, mainly from the Tampa Bay area and the Suwannee River, formed over tens of millions of years, and agatized coral has been Florida’s official state stone since 1979.
Most fossils preserve the shape of a living thing. Agatized coral preserves the shape of an entire colony, down to the tiny star-like cups where individual coral animals once lived, but every bit of the original skeleton has been swapped for quartz.
Florida’s state stone
In 1979 Florida designated agatized coral as its official state stone. It is found in several parts of the state, most famously around Tampa Bay and along rivers in the north such as the Suwannee and its tributaries, where it weathers out of limestone beds laid down in shallow seas roughly 20 to 30 million years ago.
From reef to rock
Coral skeletons are made of aragonite, a form of calcium carbonate that is not stable over geological time (see aragonite and calcite). After the reef was buried, groundwater rich in dissolved silica moved through the limestone. Silica crept into the coral and, molecule by molecule, replaced the carbonate as it dissolved. When the process was slow and even, the silica copied the coral’s structure exactly, keeping the corallites (the individual cups) and the walls between them.
The silica crystallized as chalcedony, the microcrystalline quartz that also makes up agate. That is where the name comes from. See agate and chalcedony for how that material forms.
A mineral that takes over another’s shape like this is called a pseudomorph, and agatized coral is one of the most beautiful examples. It is a close relative of petrified wood, where silica does the same thing to a tree. The pseudomorphs post explains the broader idea.
Coral geodes
Sometimes the coral dissolved faster than silica could replace it, leaving a hollow. Silica then lined the cavity from the walls inward, often with a crust of botryoidal chalcedony or tiny quartz crystals. These are sometimes called coral geodes: the outside is fossil coral, the inside is a small crystal-lined pocket.
What to look for
- Pattern: a honeycomb or flower-like pattern of small cups, visible on a broken or polished face.
- Color: white, cream, grey, tan, pink, and red to brown, with iron staining producing the warmer tones.
- Hardness: about 6.5 to 7, like other chalcedony, so it scratches glass and takes a high polish. Living coral, by contrast, is soft.
- Translucency: thin edges often glow when held to light, a sign of good chalcedony replacement.
A note on collecting
If you plan to hunt for it yourself in Florida, check current state rules first. Collecting on state lands, in parks, and in state waters is regulated, and some areas prohibit it entirely.
