pietra dura

from The Collaborative International Dictionary of English v.0.48

Pietra dura \Pi*e"tra du"ra\ [It., hard stone.] (Fine Arts)
   Hard and fine stones in general, such as are used for inlay
   and the like, as distinguished from the softer stones used in
   building; thus, a Florentine mosaic is a familiar instance of
   work in pietra dura, though the ground may be soft marble.
   [1913 Webster]