1 pint mixed berries (like raspberries, blackberries or strawberries)
2 1/2 cups sugar
1 bottle dark rum (1 liter), more as needed
1 pound peaches and nectarines, washed, pitted (pits reserved) and sliced 1/2-inch thick
1 pound mixed plums, washed, pitted and sliced 1/2-inch thick.
Place berries in a large jar. Gently toss with 1 cup sugar. Let sit for 2 hours, then top with 1/3 of the rum. Do not stir.
Cover berries with peaches and nectarines. Top with 3/4 cup sugar and another 1/3 of the rum. Do not stir.
Add plums. Top with remaining sugar and cover with rest of rum, leaving room for 1 inch of liquid on top. Do not stir.
Loosely wrap reserved peach and nectarine pits in a thick towel. Use a hammer or kitchen mallet to crack the pits and extract the kernels. Crush the kernels gently and add to the vessel.
Cover and let sit in a dark, cool place (but not the refrigerator) for at least 3 weeks and preferably 12. If the fruit floats to the top, turn the jar upside down so all the fruit falls to the bottom (you may have to do this several times over the weeks).