This is a Fossilizaiton of an Agatized Coral. Found in Withlacoochee River, Flordia. It measures approx. 81 x 55 mm: thickness is 18 mm: weight is 93 grams. The face of this piece is polished. 
 
Agatized Coral are the skeletal remains of anthozoans. These animals died some 25 to 38 million years ago. Agatized process over time is when the coral and the limestone voids are filled with silica rich water and the limestone is replaced by chalcedony quartz aka a common opal.