The green rock

When my son, Scott, was in kindergarten he was told to bring a rock from home. He found a nice one and took it to school. In art class that day they were to paint it. He painted his rock green. It must be very good paint for it has never worn off. It has been a door stop, a paper weight and just handy. Right now it is in the kitchen by the sink. In my yard there is a smart chipmunk that has found a way to get into my kitchen and eat my dog, Buster's, food. I do not like chipmunks in the house. I have locked each door with a screwdriver in the handles but have one other spot and there I have the green rock. It works. I wonder what kind of paint they used?