Looks like a Septarian Geode. A Geode has open space inside which hasn't filled in all the way. When it fills all the way up and there is no open space it is called a Nodule. A Septarian Nodule in this case :)
"Rainbow Rock"- the detail with the perspective on the side facing away is a nice touch. If only the same care had been put into making the gradient more organic. plus a small, selective color palate is much more attractive than an easy rainbow.