Remove the outer skin and seeds from the star fruit, cut the flesh into small pieces (weigh the mass in detail, correct the mass of preserving sugar if necessary).
Boil the kumquats once in salted water and drain. Cut the fruit in half lengthwise and remove the seeds.
Grind everything finely. Combine with the jelling sugar in a wide saucepan. Bring to a boil while stirring. Then make bubbling for 3 min. Immediately pour hot into prepared, carefully cleaned twist-off jars, seal, and invert lids for 5 min. Invert and cool.
Gelation test: put a little bit of jam on a cold plate, if it doesn’t set on the spot, bubble it again briefly.