Structures of Undetermined Origin

This rock is called the "undetermined structure."  My guess is that it is a rock, but I am no scientist.  One of my friends (I am looking at you, Betsy) has been on archeological digs, and probably has the best insight into the mysterious origins/uses of this surely magical rock.


Which leads me to the rock in the second picture, referred to in the guide as the "Mensal stone,"  the earliest known tampon.