These candies are just like milk chocolate M&M's, just in the shape of rocks. My grocery store sells them in their candy aisle for about $5.00 for a 12 oz package, and I've also ordered them from Fiesta Candy Company.
Here I've use the chocolate rocks to make an "S" on Sophie's 6 year-old Island Volcano Birthday Cake, and to randomly decorate the volcano and beach. Aren't they cute? They are such a super touch, and kids always want to make sure they get a piece of cake with "rocks" on it.