I find all sorts of fossils that are in a situation that I can’t describe, then someone tells me the word for it. Steinkern is one of those. It’s a fossil consisting of mud or sediment turned to a mass of stone. It is formed by entering the hollow portion of a natural object. A bivalve shell can become one easily. A great example of one can be found on my Composita post. Below is one from a Brachiopod.