before vowels petr-, word-forming element used from 19c., indicating "rock, stone" (in anatomy it refers to the petrous parts of certain bones), from Greek petros "stone, boulder," petra "rock, rocky mountain range, cliff" (see petrous).
word-forming element meaning "a speaking, discourse, treatise, doctrine, theory, science," from Greek -logia (often via French -logie or Medieval Latin -logia), from -log-, combining form of legein "to speak, tell;" thus, "the character or deportment of one who speaks or treats of (a certain subject);" from PIE root *leg- (1) "to collect, gather," with derivatives meaning "to speak (to 'pick out words')." Often via Medieval Latin -logia, French -logie. In philology "love of learning; love of words or discourse," apology, doxology, analogy, trilogy, etc., Greek logos "word, speech, statement, discourse" is directly concerned.
<a href="https://www.etymonline.com/word/petrology">Etymology of petrology by etymonline</a>
Harper, D. (n.d.). Etymology of petrology. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/petrology