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It looks like a Geode to me - http://en.wikipedia.org/wiki/Geode - I think I can see crystals inside from your picture.
Thanks for your thoughts
The inside of the object has a bumpy appearance and is darker in colour to the outside but is not crystalline.
It's not a dinosaur egg but it's hard to say what it is from the photos, especially not knowing any locality details. There are many possibilities - cavity infill of some sort, something man-made, concretions can end up hollow...