1. In a 2 1/2- to 3-quart pitcher, combine mint sprigs, rosemary sprigs, and sugar. With a long-handled wooden spoon, muddle (or crush) the herbs and sugar together. Squeeze juice from oranges into pitcher, then drop quarters in. Press a few times with spoon to release oil from orange peels. Stir in gin, lime juice, and 1/2 cup ice water. Mix until sugar is dissolved. Serve (see step 2), or cover and chill up to 4 hours (see notes). If using martini glasses, chill in freezer.
2. To serve, slowly pour chilled soda water into the pitcher, down the side. Stir gently to blend. Pour through a strainer into chilled martini glasses or ice-filled glasses. Float a mint leaf on top of each. Serve at once.
Crushed mint turns dark after about 20 minutes. If you'd like to make this drink up to 4 hours ahead, strain the mixture just before your guests arrive, return it to the pitcher, then add a handful of fresh mint leaves.