the rock, the tumbler,

ever heard the expression, you have rocks instead of brains? how would you term a person who thought about rocks all the time? Robert is polishing rocks in our tumbler. every few days he takes them out rinses them off and expects you to goo goo over them. then he puts them back for a few more days. he plans to make necklaces, bolo ties, belt buckles and other such stuff. we went on a rock hunting trip two weeks ago, so not fun or productive. and he is on another one today with his dad and two of our kids. he does'nt find pretty rocks the rocks he tumbles are brosn or grey. but he really enjoys himself.