Mystery Rock

I have an unusual rock, slightly magnetic, small round holes on the inside resembling air pockets/bubbles, similar to a meteorite, Outside looks like an underwater lava extrusion made up of 3 cylinders fused in the middle, 6 inches tall by 3-4 inches wide, exterior texture is raised horizontal ridges top to bottom, ends are broken off, was past of a longer piece, very heavy for its size, any ideas, and can i post a picture here? Thanks much