I could see where this would be a sign that he doesn't like having his knowledge or expertise called into question in any way, which for me would make me uncomfortable.

But I also think it could be a quirk. Maybe he's been a geology buff since he was 9 years old and he's just got a thing about it, and it slipped out because his geology hobby is not usually something he'd have to worry about in session.

Are you considering asking him about it, or telling him it made you uncomfortable?