The Collaborative International Dictionary
Petro-
Petro- \Pet"ro-\ A combining form from Gr. pe`tra a rock, pe`tros a stone; as, petrology, petroglyphic.
Douglas Harper's Etymology Dictionary
petro-
before vowels petr-, word-forming element used from 19c., from comb. form of Greek petros "stone," petra "rock" (see petrous).\n
petro-
word-forming element used from mid-20c. to mean "of or having to do with petroleum products," from petroleum.\n
Wiktionary
petro-
pre. 1 of or pertaining to stone 2 of or pertaining to petroleum-based products 3 (context neologism English) Pertaining to, or funded by, oil export