1. Someone who professionally studies the origin, history, structure, and composition of the Earth; a geologist. We found some kind of mineral deposit during the dig, so the university is going to send some rock hounds over to have a look at it.

2. An amateur collector of rocks, minerals, gemstones, fossils, etc.; one who collects and/or studies such things as a hobby. Great hike, Dan—nothing to see here but a bunch of boring dirt and stones. Only a rock hound like you could find this interesting!