pet·rous 
(pĕtrəs)
(pĕtrəs)adj.
1. Of, relating to, or resembling rock, especially in hardness; stony.
2. Of or relating to the very dense, hard portion of the temporal bone that forms a protective case for the inner ear.
[Middle English, from Old French petros, from Latin petrōsus, rocky, from petra, rock, from Greek petrā; see per-2 in the Appendix of Indo-European roots.]