John —, English art critic whose works include three-volume treatise The Stones of Venice (6)
I believe the answer is:
ruskin
(Other definitions for ruskin that I've seen before include "John ---, 19th Century social and art critic" , "John -, English art and social critic" , "John --, English art critic and reformer" , "John -, Victorian author, poet and artist" , "John - nineteenth century English art critic" .)