Chop the nectarines into small pieces without peeling them.
Toss the chopped nectarines with lemon juice, then stir in the sugar. The mixture will look thick and grainy at first.
Cover and refrigerate for at least 4 hours, preferably overnight, so the nectarines release their juices.
Transfer the macerated fruit mixture to a deep stock pot or Dutch oven, keeping the pot no more than half full to allow space for foaming.
If canning, prepare a water bath canner, jars, and lids before cooking the jam.
Place the pot over high heat and bring the mixture to a boil, stirring continuously.
Cook on high heat, stirring constantly, until the jam reaches its set point. Use a chilled plate test or an instant-read thermometer.
At sea level, jam generally sets at 220°F. Reduce that temperature by about 1°F for every 500 feet above sea level.
Once the jam reaches set, remove it from the heat and ladle it into prepared jars, leaving 1/4 inch headspace.
For shelf-stable storage, process the jars in a water bath canner for 10 minutes, or 15 minutes above 6,000 feet elevation.
Remove jars to a towel-lined counter and let them cool undisturbed. Check seals after 24 hours.
Store properly sealed jars in the pantry for 12 to 18 months. Refrigerate after opening.
For refrigerator jam, store for 2 to 3 weeks. For freezer storage, use freezer-safe straight-sided jars and freeze for up to 6 months.
