Rock sugar is a form of sugar composed of very large sugar crystals. People may also hear it referred to as “rock candy,” referencing one very popular use for it. Many candy stores and some markets sell the crystals for a variety of uses, and it can also be easily made at home.
When rock sugar is made, it begins with heating water and adding sugar to it in order to create a supersaturated solution of two parts sugar to one part water. In this supersaturated solution, there is so much sugar in the water that the mixture cannot remain in a liquid form, and sugar starts to precipitate out of the solution in the form of crystals. As the water evaporates, more and more sugar crystallizes, and the crystals typically grow quite large as long as the mixture is not disturbed. The end result is a cluster of very large sugar crystals.
Most companies and cooks add sticks or weighted strings to their sugar solution when they make rock sugar. These surfaces provide a space for seed crystals to start, encouraging the rest of the sugar to adhere to these crystals and forming clusters that are easy to handle. The use of strings or sticks also promotes faster crystal growth, ensuring that they are ready within a few days.
People have been making rock sugar for centuries. It appears to have originated as a candy in the Middle East, where it was often flavored with things like orange and lemon water. Sugar was once relatively rare and quite expensive, so candy made from pure sugar was a desirable sweet treat. Modern rock candy continues to be flavored and colored with an assortment of ingredients to make it more interesting, and it is a popular sweet in many regions of the world.
In addition to being used for candy, rock sugar can be used in other ways. It makes an unusual and interesting garnish for desserts, for example, and swizzle sticks coated in it can be offered with tea or coffee for an unusual twist on the traditional sugar bowl. The sugar may also be crystallized into cubes to form slow-dissolving and visually interesting sugar cubes. Some companies make the product available in a broken loose form for people who wish to sprinkle it onto scones, tarts, and other treats, and this loose form can also be added to ice cream and baked into various desserts for a textural surprise.