You could make a set just like that. Find a brush that is the right size, cut and sand a thin dowel stick so it is the correct size. The make the artifact, just find small trinkets that would work for artifacts, and paint them accordingly. Mix plaster of Paris and tempera paint to make your "rock" the rick color. Pour a bit into anything you want toy use as a mold. Once it firms up, add your artifacts, and top with more plaster.
I think I have a "Kid's Concoctions" book here somewhere with better instructions for the "rock". I will see if I can find it.