There is a pair of Fossilized Agatized Coral. One piece measures  approx. 90 x 26 mm: thickness is 8 mm: weight is 20 grams. The other piece measures approx. 67 x 20 mm: thickness 14 mm: weight is 14 grams. They were found in Tampa Bay, Florida.
 
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.