Preheat oven to 375 degrees; grease a cookie sheet. In a large bowl, combine the flour, sugar, baking powder, salt and orange peel; blend well. Using a pastry blender or fork, cut in the margarine until the mixture is crumbly. Add the egg and enough buttermilk until the mixture leaves the sides of the bowl and forms a moist dough. Stir in the prunes. To form each scone, drop 1/4 cup of dough onto the cookie sheet. Sprinkle with sugar. Bake for 13 to 16 minutes, or until the edges are light golden brown. Serve warm.