petro- (2) Look up petro- at Dictionary.com
word-forming element used from mid-20c. to mean "of or having to do with petroleum products," from petroleum.
petro- (1) Look up petro- at Dictionary.com
before vowels petr-, word-forming element used from 19c., from Greek petros "stone, boulder," petra "rock, rocky mountain range, cliff" (see petrous).