Fill two 12–ounce tall glasses halfway with ice cubes. Pour into each glass, half the pineapple juice, cucumber juice, mint tea, and lime juice. Add 1 packet Stevia to each glass, and the rum, if using. Garnish with the rosemary and mint. Serve immediately.
Note: To make cucumber juice, peel and cut 2 Persian cucumbers into ½-inch slices. Whirl in a blender with ¼ cup water until smooth. Strain the mixture through a fine strainer, pressing firmly on the solids. Tightly covered, refrigerated cucumber juice keeps for 2 days