"What Is It?" Wednesday

Alright, this one is more of a scavenger hunt.

Hint, it's made of coquina.

It marks a feature of national significance.

It's near the largest coquina fort in the world, and next door to a National Register listed cemetery.

Oh, and it's heavier than heck!

(no, I didn't really touch the coquina)

WHAT IS IT???

WHERE IS IT???