This is a piece of the stromatoporoid Stomatopora pustulifera from the stromatoporoid reef in Petoskey, Michigan, USA. It should be 380 million years old, from the Givetian epoch of the middle Devonian Period. I sanded and polished the edges down the show the growth lines and diagnostic pillars and laminae (you can see those too if you can find a dissecting microscope big enough). Also has a hole left by an epibiont or parasite that stuck itself in the upper layers and stayed there until the stromatoporoid grew around it. Very interesting mantelpiece decoration.