I found this a couple of years ago with no resolution or thoughts on what it might be.
It is astoundingly close to circular. 6 feet (1.83 meters) in diameter. Dark brown in color but with color variations. Unfortunately, I did not get any close-ups of the orb itself. It seemed fairly indeterminate as to any pattern such as corals might make.
The formation it is in is full of crinoids. The brown orb seems to have a radial pattern. Again too dark to get a decent shot while I was there.
I am sort of thinking this may be some sort of reef material? Giant stromatoporoid?