As best i can tell with my limited knowledge the description as a " nodule" is accurate. I am going to go back to where it was found and see if i can locate the source. Thanks for everyone's input and knowledge
Is it magnetic or made of metal do you think Watson did you check with a detector or magnet? I think Brian is probably correct he seems to know his stuff
I think it is natural, as well. I found a similar one shaped like a fortune cookie, with the darker color on the edges as well. Mother Nature comes up with some weird shapes!
I was going to suggest iron nodule as well based on the appearance and flow pattern your object exhibits. They often form over and around organic matter that would eventually decay. Here is an example still set in rock.
Brian nailed it; it's a concretion. If I recall right there was a beach in New Zeeland which had quite large, hollow ones.
Perhaps it's natural, albeit very cool!