Rock candy is a type of confectionery

Confectionery

Confectionery is the set of food items that are rich in sugar, any one or type of which is called a confection. Modern usage may include substances rich in artificial sweeteners as well...

 mineral

Mineral

A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

 composed of relatively large sugar

Sugar

Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...

 crystal

Crystal

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

s. This candy

Candy

Candy, specifically sugar candy, is a confection made from a concentrated solution of sugar in water, to which flavorings and colorants are added...

 is formed by allowing a supersaturated

Supersaturation

The term supersaturation refers to a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances...

 solution of sugar and water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 to crystallize onto a surface suitable for crystal nucleation

Nucleation

Nucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...

, such as a string or stick. Heating the water before adding the sugar allows more sugar to dissolve thus produces larger crystals. Crystals form after 6–7 days. Food coloring

Food coloring

Food coloring is a substance, liquid or powder, that is added to food or drink to change its color. Food coloring is used both in commercial food production and in domestic cooking...

 may be added to the mixture to produce colored candy.
Candied sugar has its origins in India

India

India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

 and Iran.