Line an 8-by-5-inch loaf pan with foil. Oil foil and set aside.
Pour 1 cup water into a bowl; stir in gelatin and let soften for 5 minutes.
Heat juice and 1/2 cup sugar until sugar is dissolved. Add softened gelatin and stir until melted. Pour mixture into prepared pan. Let stand at room temperature or refrigerate until firm, about 1 hour.
Unmold jelly slab onto a work surface and cut into 1-inch pieces with a large, sharp knife or mini cookie cutters. If desired, roll jellies in more sugar just before serving.