This a variation on common crystal growing techniques. Instead of growing crystals on a string, grow them inside a 'geode' made from an egg shell and plaster of paris.
Egg shells (washed)
plaster of paris
craft sticks or stir sticks
table salt or other crystal growing material - magnesium sulfate, borax etc.
bowl or pan
optional: food coloring for colored crystals
Step 1: Prepare 'Geode' shell
Clean egg shell. Rinse with diluted bleach water, followed by plain water. You want to kill any bacteria on the eggshell.
Mix plaster of paris according to directions on package (This is usally about 2 parts plaster powder to one part water. ALWAYS add plaster powder to water, this minimizes airborne plaster dust. Stir well and thump cup a few times to force air bubbles to the surface).
When plaster starts to thicken, spread a thin layer of plaster inside the half egg shells - about 1/4 inch or 6 mm. Try to get a thick edge of plaster along the edge of the shell for a more durable 'geode'.
Let plaster set and dry.