Materials

Plaster of Paris 
Large plastic bowl (like a big cottage cheese container) and stirrer (to mix the plaster) 
Tempera paint (liquid or powder) 
Small paper cups, an egg carton, or paper tubes reinforced with duct tape   

Instructions

This is a messy craft - wear old clothes and work outside if possible. 

Before starting, collect a large container to mix the plaster in and a few smaller ones to mold the chalk in. Paper cups and egg cartons work well for molding the chalk. - If you want to make a large chalk in a paper tube, reinforce the tube with duct tape and tape up one end before adding the plaster (unreinforced toilet paper tubes and paper towel tubes fall apart when the plaster is added).

1. Mix a small amount of Plaster of Paris with water in a sturdy container (a disposable plastic container is the easiest) - the plaster should be stiff but creamy.

2. Add a small amount of tempera paint to the plaster. Stir well for a solid color - stir only slightly for a marbled look. For an even more marbled look, add two or more colors of paint to different sides of the container and stir only partly.

3. Pour the plaster into containers.

4. When the Plaster of Paris hardens (in about 20-30 minutes), take the chalk out of the containers. 

5. It won't be completely dry for about a day, but you can use the chalk as soon as it comes out.