Begin making this dessert about three hours before you plan to eat it. It's better to freeze the mixture in two trays. If you use only one tray, the mixture will need more time to freeze.
Cut the nectarines into chunks, reserving the stones (if they have a bit of flesh on them, use them in the fruit syrup).
Combine the water, sugar and nectarine stones in small saucepan. Bring to the boil, turn down to a simmer, and cook for about 20 minutes, until the mixture has thickened a little to a syrup.
Strain the syrup through a sieve.
Stir the syrup into the chopped nectarines, then blend to a smooth puree with a handheld mixer or in a blender. Pour the mixture into two shallow trays (lamington tins work well).
Cover with foil and put in the freezer. After an hour, take out the granita and flake it with a fork. Repeat the process a couple of times. Take the granita out of the freezer about 10 minutes before serving and flake it again with a fork. Serve the granita in bowls or glasses and top with fruit if you wish.