In a large, heavy pot, combine peaches, sugar, and lemon juice. Bring to a rapid boil over medium-high heat. Place a small plate in the freezer. When the jam starts to look thick and glossy, about 15-20 minutes, spoon a small amount onto cold plate and place back in the freezer for 2 minutes. Run a finger through the jam on the plate. It should leave a clear trail, and the jam should not run immediately back into a puddle. Ladle jam into clean, dry glass jars leaving ½” headspace. Cool to room temperature and cover with lids and rings. Refrigerate if using within 3 weeks, or freeze up to 1 year.