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.

Materials:
Egg shells (washed)
plaster of paris
disposable cup
craft sticks or stir sticks
Water
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

Carefully crack an egg in half - try to save the largest part of the 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.

Step 2: Prepare Crystal Growing Solution

For one egg shell geode make about 1/4 cup of solution. Start with hot water and dissolve as much salt as you can. Stir well and heat water as needed to make a saturated saltwater solution. Keep adding salt until you get a few undisolved grains of salt in the bottom of the pan. Add food coloring if desired.

Step 3: Grow Crystals

Fill plaster egg shell with crystal solution and set in aprotected, out of the way place.
Wait a day or more. Crystals will form as water evaporates from the salt solution. Vary air temperature and observe variations in crystal size.