Such concretions are often call septarian nodules (there are technical differences between concretion and nodule, but in practice they are often interchanged).This page on concretions gives a clue to the range of sizes and forms, including a very nice photo of a septarian nodule sawn-through and polished (a third of the way down, on the right) - <a href="http://en.wikipedia.org/wiki/Concretion">http://en.wikipedia.org/wiki/Concretion</a> Note that se