This is a set of bookends commonly know as Dragon Eggs, or Septarian nodules.  These are rare specimens of red Septarian nodules from my home state of Utah.  What makes these especially interesting is the red matrix that the calcite veins form in.  Typically the Septarian nodules will form in a dark gray limestone and the red veins of aragonite and calcite will form.  This makes the set a rare and beautiful addition to any book collection.