In a saucepan, combine sugar and water. Heat over medium heat, stirring constantly, until sugar dissolves completely.
Bring the mixture to a boil, then reduce heat and simmer for 5 minutes, stirring occasionally.
Remove from heat and let cool slightly. If using, stir in food coloring and flavoring.
Pour the syrup into the prepared mason jars, leaving about 1 inch of space at the top.
Tie a clothespin to the top of each skewer to prevent it from sinking too far into the syrup. Submerge the skewers in the jars, ensuring they don't touch the sides or bottom.
Cover the jars loosely with lids or plastic wrap and let them sit undisturbed in a cool, dark place for 7-14 days, or until crystals have grown to your desired size.
Once the crystals have grown, carefully remove the skewers and let the rock candy dry completely before enjoying!
Notes
For best results, use pure granulated sugar and avoid disturbing the jars during the crystal growing process.