Rock cany (also called rock sugar) is a type of confectionery composed of relatively large sugar crystals. Homemade rock candy is commonly formed by allowing a supersaturated solution of sugar and water to crystallize onto a string or some other surface suitable for crystal nucleation. Heating the water before adding the sugar allows more sugar to dissolve and thus produces larger crystals. Crystals form after several days. Food coloring is often added to the mixture to produce colored candy.