This Agatized Coral Fossil was found in Tampa Bay, Florida. It measures approx. 140 x 49 mm: its thickness is  variable but approx. 36 mm.  weight is 144 grams.
 
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.