Crystals can form on the jelly as a result of using excess sugar. This can be prevented by using a jelmeter to test the fruit juice for proper proportions of sugar.
Crystals can form as a result of undissolved sugar sticking to the sides of the saucepot.
This can be prevented by wiping the sides of the pan free of crystals with a damp cloth before filling the jars. There could be tartrate crystals in the grape juice that you use to make the jelly with. You should extract the grape juice and allow tartrate crystals to settle out by refrigerating the juice overnight. Then strain the juice to remove any remaining crystals before making the jelly. If the mixture is cooked too slowly or too long, crystals could easily form.