petro- 1

early 15c., "petroleum, rock oil, oily inflammable substance occurring naturally in certain rock beds" (mid-14c. in Anglo-French), from Medieval Latin petroleum, from Latin petra "rock" (see petrous) + oleum "oil" (see oil (n.)). Commercial production and refinement of it began in 1859 in western Pennsylvania, and for most of the late 19th century it was produced commercially almost entirely in Pennsylvania and western New York.

Petroleum was known to the Persians, Greeks, and Romans under the name of naphtha; the less-liquid varieties were called [asphaltos] by the Greeks, and bitumen was with the Romans a generic name for all the naturally occurring hydrocarbons which are now included under the names of asphaltum, maltha, and petroleum. The last name was not in use in classic times. [Century Dictionary, 1895]

c. 1400, in anatomy, "very hard, dense," from Old French petros (Modern French petreux) and directly from Latin petrosus "stony," from petra "rock," from Greek petra "rock, cliff, ledge, shelf of rock, rocky ridge," a word of unknown etymology (Beekes says it is "probably Pre-Greek"). Used of certain bones, especially of parts of the temporal bone.