Yes, Yemaya is right....I have had hollow stones, too...and my theory is that the longer they stay inside you, the faster the insides melt! yukky. Did you notice a particularly awful smell? The insides of the stones have melted out into your bowel...I think that is one reason to really make sure that you flush yourself out with the Epsom Salts ...because you don't want those toxins to get into your bloodstream! ick.
Isn't it kinda cool, though? Because you can really see a CRUST.
Anyway...glad you had such a good one and that your yellowness is going away! (they're going to have to think of another nickname! ha.)