petro-, petr-, petri-, peter- (Greek > Latin: stone, rock).
"Essence of rock." The pleasant odor from falling rain on dry ground; especially after a long period of warm, dry weather. The odor is said to be a result of an oily essence that comes from rocks or soil when rain falls on them. It is believed that the oils are given off by vegetation during dry periods and are adsorbed on to the surface of rocks and soil particles which are then spread into the air by the next rain. Based on Greek petro plus Greek ichor, "fluid that flows like blood in the veins of the gods."