In a pot, combine the water and sugar over medium heat and stir until the sugar dissolves. Remove from heat and stir in the gelatin.Add the lime juice and 2 mint stems (with leaves). Allow that to steep for 12-15 minutes. Remove the mint stems and stir in the rum if using.
In the meantime, hollow out your lime shells. This is probably the hardest part of making these.I used a small sharp knife to carve around the inside of the limes and then a mellon baller to scoop out the centers. If the insides don’t look perfect don’t worry! Mine looked far from perfect. Be sure to leave the limes cut in halves until after the Jello has set, then you can slice them into wedges.
Fill each lime shell with the Jello mixture and sprinkle in some chopped fresh mint (from the remaining leaves on that 3rd stem). Place in the refrigerator and allow to set for at least 3 hours.