The skins from stone fruit will contribute color and flavor to the jam. But for a smoother mixture, you can start by peeling peaches or nectarines: Carve an X in the bottom of each and plunge them into boiling water for 30 seconds, then transfer them to an ice-water bath to stop the cooking; the skins will slip off. For plums, just lift the skins out of the cooked jam with a fork.
1 pound each raspberries, blackberries, and strawberries
2 1/4 pounds nectarines
12 ounces raspberries
3 pounds white or yellow peaches, peeled (optional)
1 1/2 pounds each peaches, peeled (optional), and plums
3 pounds plums
3 pounds raspberries