These paintings are famous for a variety of qualities which have been much imitated by students and discussed at great length by connoisseurs and critics. Among the qualities that make Leonardo's work unique are the innovative techniques which he used in laying on the paint, his detailed knowledge of anatomy, light, botany and geology, his interest in physiognomy and the way in which humans register emotion in expression and gesture, his innovative use of the human form in figurative composition, and his use of the subtle gradation of tone. All these qualities come together in his most famous painted works, the Mona Lisa, the Last Supper and the Virgin of the Rocks.