Can you help me discern what this is? Fossil or rock?

[Bonnie Meyer]

Found this on Tampa Bay.  Might be nothing, but looks interesting.  Hope the pictures are visible enough 

[Fossildude19]

This might be a fragment of shell, either gastropod or bivalve, with some matrix adhering to it.

Unfortunately, I don't think there is enough there to say definitively what it is.  

[val horn]

I think Tim is probably correct but I would like to see straight on the smooth circular piece to see if there is anything definitive visible.  A circle with a dimple would make me check for echinoderm.

[Holly P]

That appears to be “Tampa Bay Coral”, an agatized form of a fossilized coral typically found on Tampa Bay, but also some outlying areas.  It can be found in a variety of sizes and shapes.  I have found 2 medium size pieces on Manasota Key in Englewood. Here is a picture with the 1 being cut open to show the agate inside