If your candy is crumbly or sandy instead of hard and crunchy, or opaque instead of translucent, then it has crystallized during the cooking process. Nothing is wrong with it, and it will still taste fine, but the appearance and texture may not be what you want. Crystallization happens when sugar crystals form during the cooking process. This can happen due to stirring too much while it cooks (once it comes to a boil, you should not be stirring it at all), or not wiping down the sides of the pan with a wet brush to remove the crystals, or just accidentally having stray crystals on the candy thermometer, spatula, or pan. The corn syrup in the recipe helps prevent crystals from forming, so if you substitute other ingredients for the corn syrup, you might find that crystallization is more likely.