Wash nectarines well. Pit and quarter them; no need to peel. Place in a large stainless steel saucepan.

Add about 1/2 cup water per pound of nectarines. Bring to a boil over high heat, stirring frequently. Reduce heat, cover loosely, and boil gently, stirring and crushing occasionally, until softened, about 20 minutes. Do not overcook.

Transfer to a dampened jelly bag or cheesecloth-lined strainer set over a deep bowl. Let drip undisturbed for at least 2 hours or overnight. Do not squeeze.

In a large saucepan, combine nectarine juice and sugar. Heat over medium-high heat, stirring constantly, until sugar dissolves. Bring to a boil, reduce heat, and simmer 5 minutes. Stir in lemon juice if using.

Ladle hot syrup into prepared jars, leaving 1/4 inch headspace. Wipe rims, center lids, and apply bands to fingertip tight.

Process in a boiling water bath canner for 10 minutes for pints or half pints, adjusting for altitude. Let stand in canner 5 minutes before removing.