'petrological' derives from the noun 'petrology', which comes from Greek 'petros' meaning 'rock' and Greek 'logia' meaning 'study' (via Modern Latin/Scientific Latin).

'petros' (Greek 'rock') combined with '-logia' produced Medieval/Modern Latin 'petrologia', which entered English as 'petrology'; the adjective form 'petrological' was formed in English by adding the suffix '-ical' to 'petrology'.

Initially it denoted 'of or relating to petrology (the study of rocks)'; over time it has continued to be used both for the scientific discipline and for describing rock characteristics, so the core meaning has remained stable.