Combine diced nectarines with sugar and lemon juice and stir well. Allow to sit, lightly covered, for up to 8 hours. The sugar and lemon will draw the juice out of the nectarines.
Pour the nectarines, sugar and lemon juice into a large stock pot. Bring to a boil until the sugar completely dissolves. For a smoother jam, use an immersion blender to break up the larger nectarine pieces.
Wash the canning jars. Place the canning jars, lids and bands in the oven at 200 degrees F to sterilize while you are making the jam (it helps to have them all in an 13x9 dish to keep them all together).
Bring to a rolling boil until the temperature of the mixture reaches 220 degrees F (105 degrees C). Allow to boil for 2-3 additional minutes.
Remove from heat and ladle into sterilized canning jars, leaving 1/2 inch headspace. Wipe rims thoroughly and then apply lids and bands (finger tight).
Placed filled jars in a hot water bath, ensuring that they are covered by 2 inches of water. Place lid on water bath canner, and bring water to a steady boil.
Process the jars for 10 minutes (half pint), adjusting for altitude. Boil 10 minutes at altitudes of 1,000 feet or less. Boil one additional minute for every 1,000 feet elevation.
Turn off the heat, remove the lid and allow the jars to rest for 5 minutes. Then remove the jars entirely and allow to cool.
Check lids for seal after 24 hours. Lids should not flex up and down when center is pressed. Store jars in a cool place for up to 1 year.