To make hard rock candy, mix water and sugar and heat the mixture until it reaches hard-crack stage at 300 degrees. Once it does so, remove from heat, add flavoring, pour the mixture into a lined pan and let it cool.
The Taste of Home website has a recipe for cinnamon rock candy. It calls for 1 cup water, 3 3/4 cups sugar, 1 1/4 cups light corn syrup, 1 teaspoon cinnamon oil, 1 teaspoon red liquid food coloring and 1/3 cup powdered sugar.
Mix all the ingredients in a large saucepan and heat on medium. Stir occasionally as you bring the mixture to a boil. Cover the pan and boil for three minutes to allow the sugar crystals to completely dissolve. Uncover the pan and heat the mixture for about 25 minutes over medium-high heat until a candy thermometer reads 300 degrees, indicating it has reached hard-crack stage. Remove from the heat and add the cinnamon oil.
Pour the candy mixture into a 15-by-10-inch pan lined with buttered tinfoil and let it cool for at least 45 minutes. Once cooled, break the candy with a kitchen mallet and dust with powdered sugar. This recipe makes about 2 pounds of candy, equal to about 32 servings.