Domestic plums are crimson to black-red with a yellow or reddish flesh; they are in season May through October. European varieties, which peak during the fall months, range from green to blue to purple and have a golden yellow flesh.
The skins from stone fruit will contribute color and flavor to the jam. But for a smoother mixture, you can start by peeling peaches: 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.
Food editor Shira Bocar's drink of choice for summer evenings on Decatur Island is a play on the Negroni, featuring cherry plums muddled with sugar, Aperol (a citrusy Italian aperitif), gin, seltzer, and a twist of orange.