First, clean a glass jar by washing it with hot water. Get a thick cotton thread that is a bit longer than the height of the jar, and tape it to a pencil.
Put the pencil on top of the jar with the thread suspended around an inch from the base of the jar. Fasten a paper clip at the hanging tip of the thread to serve as a weight and make the thread hang straight down. You can also have a wooden skewer as the supporting structure of your rock candy. Use a clothespin placed on top of the jar to clip it into place.
Roll a wet thread or wooden skewer in granulated sugar to make a base layer where the sugar crystals will attach as they form. Set the base aside as you begin the preparation of your sugar syrup.
Bring 2 cups of water to a boil in a medium-sized pan. Add 4 cups of sugar, one cup at a time. Stir the syrup after adding each cup. As you do this, you will observe that it takes more time for sugar to melt after every addition. Go on with stirring and bringing the syrup to a boil until you have emptied the four cups and all these have been dissolved. Then remove the pan from the heat.
Flavorings or colorings should be added at this point. Add only ½ teaspoon or 2.5 ml of flavoring oils. Add a teaspoon of extract to the syrup. As you add the flavoring, don’t stand directly in front of the pan because the smell that comes along with the rising steam can be very strong. Should you wish to add food coloring, add 2-3 drops and stir well to make sure there is even color.
Let the sugar syrup cool for 10 minutes, before emptying it into the jar. Lower the skewer or sugared string until it is suspended about an inch from the bottom. Cautiously transfer the jar in a cool place away from lights. Loosely cover the jar with paper towel or plastic wrap.
In 2-4 hours, the sugar crystals will start forming. Allow the rock candy to grow into the size that you want. Once the desired size is reached, take it away and allow it to dry for a few minutes. Then wrap the candy for consumption.