I found this thing on the beach...Does anyone have any clue what it would be?

I found this when I was in Jupiter, Florida (part of West Palm Beach). It is a small (smaller than the size of a quarter), grey with a dark brown "tip" at the end (about the size of a pin head) and there are some fine darker grey lines that circle the "Thing". It is extremely hard and could not be opened or scratched by hitting it on a rock, and it appears to have something in it.

Any Ideas?