Rock candy is a type of confectionery composed of relatively large sugar crystals.
Homemade rock candy is commonly formed by allowing a solution of sugar and water to crystallize onto a string or some other surface. Superheating the solution by boiling the water before adding the sugar can help produce larger crystals. Crystals form after several days. Food coloring is often added to the mixture to produce colorized candy.
Rock candy is a different product than British rock, also called "seaside rock", which more closely resembles a candy cane. The word "candy" is not usually used for confectionery in the UK.