Pietra dura(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.
Webster's Revised Unabridged Dictionary
It., hard stone
PIETRA DURA, a name given to the purest kind of Florentine mosaic work, consists of hard stones characterised by brilliancy of colour.
"The Nuttall Encyclopaedia" by Edited by Rev. James Wood