Chunks of crystal sugar, often sold on a stick. You can make rock candy at home, here's how:
Heat a pot of water to just below simmering, and start adding sugar. Keep adding sugar and stirring until the sugar no longer dissolves completely in the water. (At this point, the water is saturated with sugar.) Pour off the liquid into a jar or pitcher, but make sure there's no solid sugar or it will crystallize on the bottom of the pitcher and that's not what we want to happen. Roll a string in the undissolved sugar, and let it dry. Suspend the string in the pitcher; soon the seed crystals on the string will start growing into rock candy. If you can leave the pitcher in a warm, dry place, perhaps covered with cloth, but not sealed, the water will evaporate, forcing the crystals to grow faster.