I haven't tried these and I can't remember where I found them, but they look like fun:

water
borax*
clean egg shells halves
egg carton
waxed paper

Create a super-saturated solution of borax and water -- fill a jar with boiling water then add borax one tablespoon at a time until no more will dissolve. This will be about three tablespoons per cup of boiling water. Place small pieces of waxed paper in the egg carton sections. Set the clean egg shell halves in the carton on top of the waxed paper. Pour a small amount of the super-saturated borax solution into the egg shell halves. In a few days, or less, crystals will form inside the shell.

*You can use other solids to make super-saturated solutions such as: table salt, rock salt, sugar, baking soda, and Epsom salts. Each of these will create a different looking crystal and will require a different ratio of water to solid, and will require a different number of days to form. Makes an interesting science project.

Edible Rock Recipes
Igneous Rocks
1 can evaporated milk (6 oz.)
1-1/2 c. sugar
1/4 tsp. salt
1/2 c. chopped nuts
1-1/4 c. mini-marshmallows
1-1/2c. chocolate chips
1 tsp. vanilla

Grease an 8" or 9" pan. Combine evaporated milk with sugar and salt in a saucepan. Bring to a boil. Reduce heat and simmer, stirring for 5-9 minutes. Remove from heat. Add remaining ingredients and stir gently. Pour into prepared pan. Let cool. (refrigerate) 1 pan (8") makes 30 (1") squares.

Metamorphic Rocks
1/2 c. butter
1 c. brown sugar
1/2 tsp. vanilla
1/2 c. chopped nuts
1/2 c. crushed pineapple, well drained
1 egg, beaten
1 c. whole wheat flour
1 c. flour
1 tsp. soda
1 tsp. salt

Preheat oven to 375°. Cream the butter and sugar. Add the beaten egg and vanilla. Stir in the dry ingredients and mix well. Stir in the fruit and nuts. Drop by tsp. onto cookie sheet and bake for 7 min. at 375°. (chocolate chips, coconut, wheat germ, etc. can also be added)

Sedimentary Rocks
1-1/4 c. crushed flake cereal
1-1/2 tbsp. sugar
1/2 c. margarine
1 c. chocolate or butterscotch chips
1-1/4 c. coconut
1 c. chopped nuts
1 can sweetened condensed milk (14 oz.)

Preheat oven to 325°. Melt the butter. Mix with the cereal and sugar. Press into an 8" or 9" pan. Add the following (in layers): chips, coconut, and nuts. Drizzle the sweetened condensed milk over all. Bake for 25 minutes at 325°.