petrous (adj.)

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.