The skins from stone fruit will contribute color and flavor to the jam. But for a smoother mixture, you can lift the skins out of the cooked jam with a fork.
Stir together plums, sugar, and 1/4 teaspoon salt in a large heavy-bottomed pot. Bring to a boil, stirring until sugar is dissolved and mashing plums with a potato masher. Add lemon juice; continue to boil, stirring frequently, until bubbles slow, chunks of fruit show at top, and mixture clings to a spoon but falls off in clumps, 10 to 12 minutes. Skim foam from top.
Ladle jam into clean containers, leaving 3/4 inch of headroom. Let cool completely. Cover, label, and refrigerate up to 1 month, or freeze up to 1 year.