Prepare the water bath canner by filling it with water and bringing it to a simmer (180°F). Sterilize canning jars, lids, and rings according to standard water bath canning procedures.

In a large, heavy-bottomed pot, combine the chopped nectarines and lemon juice. Heat over medium heat, stirring occasionally, until the fruit starts to release its juices and simmers (about 10-15 minutes).

In a small bowl, whisk the powdered pectin into 1/4 cup of the measured sugar until completely smooth. Add this mixture to the simmering fruit, stirring constantly for one minute.

Gradually add the remaining sugar to the fruit mixture, stirring until it dissolves. Increase the heat to high and bring the mixture to a rolling boil that cannot be stirred down. Boil hard for exactly 1 minute, continuing to stir constantly.

Immediately remove the pot from the heat. Skim off any foam that rises to the surface using a metal spoon. Carefully ladle the hot jam into the sterilized jars, leaving 1/4 inch of headspace in each jar.

Wipe the rims clean with a damp cloth, center the lids, and screw on the bands fingertip tight. Place the filled jars into the simmering water bath canner, ensuring they are covered by at least 1 inch of water. Process for 10 minutes (adjust time for altitude if necessary).

Carefully remove the jars and place them on a towel-lined counter to cool undisturbed for 12-24 hours. Check seals; any unsealed jars should be refrigerated and consumed within three weeks.