Combine the flour, granulated sugar, baking powder and salt in a medium bowl. Zest the lime with a grater and add 1 teaspoon of the zest to the bowl.
Cut the lime in half and squeeze the juice from one half into a small bowl. Reserve the other half of the lime for later. Add the milk to the bowl with the lime juice. The milk will curdle, producing buttermilk!
Add the finely cut butter to the flour mixture and gently rub it with your hands or use a pastry blender until it is coarse and mealy; you don't want to work it in completely. Stir in the frozen cranberries.
Whisk the egg into the milk mixture in the small bowl. Add the liquid from the small bowl into the butter-flour-and-cranberry mixture, folding until the dough just comes together.
On a lightly floured surface, press the dough into a 1-inch-thick round disk. Cut the disk into 8 slices.
Grease or spray a heavy baking sheet with butter or nonstick spray. Evenly space the dough wedges on the sheet. Squeeze the remaining lime half over top of each wedge to create a "glue," then sprinkle with the raw sugar. Bake, rotating the sheet halfway through, until the tops of the scones are golden brown, 20 to 25 minutes.