There are all sorts of things that kids collect. Zoe collects rocks. I find them in her pockets, under her bed, in her back pack (she currently has about 10 that she insists on carrying around with her). Zoe even had a large collection in China that she kept in a plastic bag under her bed. I had to convince her to leave it behind because of the weight. I am not sure what her criteria is for a worthy rock is because some of the rocks are just chunks of concrete. I did use a couple of them to fill a flower arrangement.
I think her brother picked up on her rock collecting because when I pulled back the covers two nights ago to go to bed, the rocks Teac had been playing with during the day -that I think he found in his sister's room some where- were waiting for me.