Carveable Stone

3 parts vermiculite (soil found at garden stores)
2 parts plaster of Paris
2 scoops water

Mix vermiculite, plaster of Paris and water. pour mixture in a paper container such as a milk carton. Wait fifteen minutes. Peel away paper. The "stone" will be soft. Mold with hands or carve using a spoon. If you want to work further on your carving, keep it in a sealable plastic bag.

IDEA: Make a secret treasure holder that looks like a rock.