Cut a wooden skewer to about 7 inches long. Spray half of the skewer with water.(You can also dunk it in a glass of water or put it under the tap.)
Sprinkle sugar over the wet part, rotating the skewer to cover it. Adding sugar to the skewers gives seed crystals for your rock candy to grow on.
Repeat with 4 other skewers and then set them on a parchment lined tray to dry.
Add gel food colouring to the end of a stir stick, then place the coloured stir stick in an empty mason jar. Repeat using stir sticks with different colours for each jar.You can also use regular food colouring. Simply add about 5 drops to the bottom of each jar.
Bring the water to a boil in a large pot.
Add 1 cup of the sugar.
Stir until the sugar dissolves.
Repeat this process with the remaining 9 cups of sugar.Add 1 cup, stir to dissolve, add 1 cup, etc. It will take longer to dissolve the more sugar you've added.
Allow the syrup to simmer for about 5 - 10 minutes. This will thicken the syrup a bit and ensure that all the sugar has dissolved.
Remove the pot from the heat (CAREFUL: it's heavy and very hot!) and stir in the candy flavouring oil.(You can also use flavouring extracts. For these, use at least 1 teaspoon.)
Use a ladle or transfer the syrup to a measuring cup and pour it into the prepared jars with the stir sticks in them. We found it too heavy and difficult to pour right from the pot.
Stir well with the stir stick, until the colours are fully combined with the syrup. Remove the stir sticks.
Allow the syrup to cool for 20 - 30 minutes.You can't add the skewers right away, or the seed crystals on them might melt right off.
Once the syrup has cooled down add the prepared skewers into the syrup. Make sure the bottom of each skewer is at least 1 inch above the bottom of the jar.Secure the skewers with a wooden clothespin and center them in the jar.
Set the jars in a place they won't be disturbed, away from any heat source or direct sunlight. Once the syrup is completely cooled, loosely cover the top of the jar with plastic wrap.
Allow the crystals to grow for 5 - 7 days. (Or longer - see more information below.)
Once your rock candy has grown to the size you want, use a spoon - or fork - to crack the crystals that have grown on the surface of the syrup.
Remove the rock candy from the jar. At this point it will be dripping sugar syrup.
Hang your rock candy inside a clean glass or jar to allow the extra syrup to drip off.
Wait between 30 minutes to 1 hour for the rock candy to fully dry.
Remove the clothespins and your rock candy is ready to enjoy!
Video
Notes
If the rock candy grows too large it may connect with the crystals at the bottom of the jar. Make sure your skewer is at least 1 inch above the bottom of the jar, and remove the rock candy before it gets too big.
You can also use lollipop sticks or popsicle sticks to grow your rock candy.