What is pietra dura?

Pietre dure is an Italian plural meaning "hard rocks" or hardstones; the singular pietra dura is also encountered in Italian. In Italian, but not in English, the term embraces all gem engraving and hardstone carving, which is the artistic carving of three-dimensional objects in semi-precious stone, normally from a single piece, for example in Chinese jade. The traditional convention in English has been to use the singular pietra dura just to denote multi-colored inlay work.