Lightly grease a 9×13 inch baking pan or line it with parchment paper. Set aside.

Combine the sugar, corn syrup, water, and cream of tartar in a heavy-bottomed saucepan. Stir gently over medium heat until the sugar dissolves completely. Do not stir once the mixture boils.

Attach a candy thermometer to the side of the pan, making sure the tip is submerged but not touching the bottom.

Boil the mixture without stirring until it reaches 300 degrees Fahrenheit (the hard crack stage). Watch the temperature closely as it rises quickly near the end.

Remove the pan immediately from the heat once 300 degrees Fahrenheit is reached. Wait about 30 seconds for the bubbling to subside slightly.

Carefully stir in the cinnamon oil and the red food coloring, if you are using it. Stir just enough to combine the flavor and color.

Quickly pour the hot candy mixture into your prepared pan. Do not spread or scrape the sides of the pan, as this can cause crystallization.

Allow the candy to cool completely and harden at room temperature for at least 2 hours, or until fully set. Do not refrigerate.

Once hard, place the candy slab on a clean surface or inside a sturdy plastic bag. Use a heavy object, like a meat mallet or the back of a spoon, to break the candy into small, irregular pieces (rock candy).

Store your homemade cinnamon candy in an airtight container at room temperature.