Prepare the Jars: Ensure all canning jars, lids, and bands are thoroughly washed and sterilized according to standard canning procedures. Keep them warm until ready to fill.

Combine Fruit and Sugar: In a large, heavy-bottomed pot, combine the chopped nectarines, granulated sugar, and lemon juice. Stir gently to mix. Let the mixture sit at room temperature for at least 30 minutes (or up to 2 hours) to allow the sugar to draw out the juices from the fruit.

Cook the Jam Base: Place the pot over medium-high heat. Stir constantly until the sugar is completely dissolved. Bring the mixture to a rolling boil that cannot be stirred down. Add the pectin according to package directions, usually stirring it in briskly after the initial boil is reached.

Achieve Setting Point: Once the pectin is added, continue to boil vigorously, stirring frequently to prevent scorching. If using, add the small knob of butter now to reduce foam. Cook until the jam reaches 220°F (104°C) on a candy thermometer, or pass the plate test (a small dollop dropped onto a chilled plate wrinkles when pushed).

Jar and Process: Remove the pot from the heat. Skim off any remaining foam from the surface. Carefully ladle the hot jam into the prepared, warm jars, leaving 1/4 inch headspace. Wipe the rims clean, place the lids on fingertip tight.

Water Bath Canning: Process the sealed jars in a boiling water bath canner for 10 minutes (adjusting for altitude if necessary). Turn off the heat, let the jars rest for 5 minutes, then remove them to a wire rack to cool completely. Listen for the satisfying 'pop' as the jars seal.