From Schoenberg to Pick-Nevanlinna: Toward a complete picture of the   variogram class
  We show that a large subclass of variograms is closed under products and that some desirable stability properties, such as the product of special compositions, can be obtained within the proposed setting. We introduce new classes of kernels of Schoenberg-L\'{e}vy type and demonstrate some important properties of rotationally invariant variograms. 
Emilio Porcu, Ren\'e L. Schilling