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originally posted by: schuyler
It looks to me like an igneous rock surrounded by a sedimentary rock. I can't quite tell because you gave us no "hardness' indication, but that's what it looks like. So the had rock was lying in a place where sediment surrounded it and hardened. Likely it was completely surrounded by the sediment. Afterwards, the combined piece was subject to erosion, in this case likely by wave action, hence your very cool rock.