Bring boiling-water canner, half full with water, to simmer. Wash jars and screw bands in hot soapy water; rinse with warm water. Pour boiling water over flat lids in a saucepan off the heat. Let stand in hot water until ready to use. Drain well before filling jars.
Pit and finely chop nectarines. Measure exactly 4½ cups (1065ml) of prepared fruit into a large heavy-duty pot (6-8 quarts). Add lemon juice; stir.
Stir in the pectin. Add the butter to reduce foaming. Bring mixture to a full rolling boil (a boil that doesn't stop bubbling when stirred) on high heat, stirring frequently.
Stir in the sugar. Return to a full rolling boil and boil exactly 1 minute, stirring frequently. Adjust the heat down a bit if the mixture rises too close to the top of the pot. Remove from heat. Skim off any foam on the surface with a metal spoon.
Ladle the jam immediately into prepared jars, filling to within ¼ inch of the tops. Wipe jar rims and threads. Cover with two-piece lids. Screw bands tightly.
Place jars on elevated rack in canner. Lower rack into canner. (Water must cover jars by 1 to 2 inches. Add boiling water, if necessary.) Cover; bring water to gentle boil.
Process for 10 minutes. Remove jars and place them upright on a towel to cool completely. After the jars cool, check the seals by pressing the middle of the lids with a finger. (If lids spring back, lids are not sealed and refrigeration is necessary.)