I am aware shells were used ornamentally and widely traded, so would expect them to show up in unusual places. They are a little more degradable, however, than stoneware.
I remember breaking up some heavy clam shells on east coast vacations as a kid, but can't recall the fracture patterns. None of my breakage was in any way constructive though. Got in a bit of trouble when I left some live ones in my sand bucket that went into the trunk. The rig was quite aromatic for several days.
