Ron and Hermione both get jealous on numerous occasions but Hermione's jealousy peaked in HBP. She was jealous of Lavender because she was dating Ron and she was jealous of Harry because he was performing better than her. She takes it to the extreme by harming Ron and attempting to make him jealous. She was so eager to catch Harry cheating just because he was performing better than her, and remember, this was before she found out Harry actually was cheating.
