Pour the black sanding sugar onto a small plate. Run a key lime slice around the rim of each of the four glasses, then dip the rims into the sugar to coat them evenly.
Fill a cocktail shaker with ice. Add the gin, key lime juice, and simple syrup.
Shake the mixture vigorously for 15 seconds until it is well chilled.
Strain the drink into the prepared glasses. Add one key lime slice to each glass as garnish.