Unidentified rock - can anyone help??

This cheeky chappie was given to me by a great uncle about ten years ago, with another specimen of the same size and shape, which I have not photographed. The provenance is most probably from the former Soviet Union or Iran - great aunt was a diplomat based in both Moscow and Teheran at some point in the '70s and '80s. They went to other places as well, but these are the most probable countries of origin for the rock.

It's heavy rock, and displays ribbing here and there and what seem to be growth rings, on close inspection. I figure it is some sort of coral, but I know very little about coral and the like.

I’d be grateful for any hints as to what this is and how long ago it used to live. If there is something I should do to aid recognition, please someone let me know.


Here is the front face:





Here is the back side (showing the vertical ribs):






This is it side-on, standing up (some of the concentric rings are visible):




OK, here you can see the vertical ribbing more:




And here again the broader ribbing - seems to be two sizes of little ribs, those about 10-15mm wide and those about 1-2 mm wide.




This is the top (or bottom) end showing, though not very clearly, concentric internal structure:




Any help gratefully received...

Cheers,