ScienceBob.com has a recipe for rock candy that requires a wooden skewer or chopstick, one cup of water, two to three cups of sugar, a clothespin and a narrow jar. This recipe, which also doubles as a science experiment, takes approximately one week to complete.
First, attach the wooden skewer to the clothespin so that it hangs about 1 inch from the bottom of the jar. This skewer serves as a "home" for the rock crystals to grow. When finished, put this aside, and keep the jar open.
Add the cup of water to a pan and bring it to a boil. Mix in the sugar slowly, continuing to add until the water is no longer able to dissolve it. After reaching this point, let the water cool for approximately 20 minutes.
Then carefully pour this solution into the jar. Make sure that the skewer hangs down, avoiding all sides of the jar. If you prefer your rock candy to have color, add some food coloring to the mix. After that, the jar needs to cool for up to a week in a low-traffic area of the kitchen.
Within a week, rock crystals begin to form inside the jar and are ready to eat.