1. Combine water and sugar in small saucepan. Bring to a simmer over medium-low heat. Cook, stirring occasionally, until sugar is dissolved, 5 minutes. Transfer to small bowl and cool completely. Refrigerate until cold, 30 minutes.
2. Meanwhile, place a metal 13x9-inch baking pan and a metal spoon in the freezer.
3. Puree grapes in food processor or blender. Strain puree through a fine-mesh sieve set over a large bowl, pressing solids with a rubber spatula to extract juice; discard peel. Stir in grape juice and chilled sugar syrup. Pour mixture into frozen pan.
4. Freeze grape mixture until ice crystals from around edges of pan, about 30 minutes. Stir ice crystals with the frozen metal spoon into center of mixture. Freeze mixture 2 to 2-1/2 hours more, stirring every 30 minutes, until all of the liquid is frozen.
5. Scoop granita into 6 small serving bowls, and garnish with grapes and mint sprigs, if desired. Makes 6 servings.