Line a baking sheet with foil. Combine granulated sugar and water in a small, heavy saucepan. Bring to a simmer and cook, without stirring, until the syrup is a deep amber color, 5 to 10 minutes. Do not let the syrup burn. Pour the hot syrup onto the prepared baking sheet. Quickly tilt to spread the caramel in a thin layer. Sprinkle on the toasted nuts. Let cool. When the caramel is hard, break into shards.