In small saucepan, combine 1/2 cup of water, gelatin, lime juice, and lemon juice. Set aside.
In heavy small saucepan, place over medium heat, combine remaining 3/4 cup of water and 1 cup sugar and stir until sugar dissolves. Bring to a boil, cover and cook for 3 minutes. Remove lid and place candy thermometer on side of pan and cook until it reaches 300 degrees F. Remove from heat, add to gelatin mixture, return pan to low heat and stir constantly in order to dissolve gelatin completely. Add lemon and lime zest and stir to combine. Pour mixture into greased 8-inch by 8-inch pan and cool to room temperature. Do not refrigerate.
Once cooled, cut into cubes and toss to coat in the remaining sugar. Store in airtight container for up to 4 days.
Recipe courtesy of Alton Brown