In a medium bowl combine crushed pineapple with the juice and unflavored gelatin. Stir until the gelatin is dissolved in the pineapple juice.
Add cranberry sauce and drained mandarin oranges, stir well. Add the zest of one orange and stir.
Place relish in a pretty glass serving dish and place in the refrigerator. Allow relish to chill for at least 30 minutes. If you prefer a sweeter relish you can add a bit of sugar to the pineapple and gelatin mixture.