2 pounds small plums (such as Italian prune plums), pitted and cut into
1 1/2 cups sugar
1/2 lemon, seeded
In a large, nonreactive saucepan, toss the plums with the sugar and let stand, stirring occasionally, until the sugar is mostly dissolved, about 1 hour.
Squeeze the lemon over the plums, add it to the saucepan and bring to a boil, stirring until the sugar is dissolved. Cook over moderate heat, stirring, until the liquid runs off the side of a spoon in thick, heavy drops, 20 to 25 minutes. Skim off any scum that rises to the surface of the jam.
Discard the lemon and spoon the plum jam into three 1/2-pint jars, leaving about 1/4 inch of space at the top. Close the jars and let the jam cool to room temperature. Store the jam in the refrigerator for up to 3 months.