Try this I Want More Jello recipe, or contribute your own.
Adding heavy canned fruit and berries directly into warm gelatin before it reaches the correct stage of thickening causes all the solid ingredients to sink straight to the bottom of the dish. Achieving an even suspension requires chilling the gelatin base until it reaches the consistency of unbeaten egg whites before folding in the drained mandarin oranges, cherries, and raspberries.
Gelatin desserts trace their lineage back to elaborate Victorian molded aspics and sweetened jellies that showcased technical prowess through clarity and suspension. Careful temperature management remains the defining variable between a cohesive fruit mold and a separated layer of fruit sitting beneath weak jelly.