Toss the rose petals, lemon juice, and 1 cup sugar together in a bowl until the petals are evenly coated. Let stand at room temperature overnight.
Bring the water to a boil in a saucepan over medium-high heat. Stir in the rose petal mixture and reserved 1 cup of sugar; stir until the sugar has dissolved. Reduce the heat to medium-low, and simmer 20 minutes. Increase the heat to medium-high, and return the mixture to a boil for 5 minutes. Stir in the pectin, and boil for 1 minute.
Pour the jam into 4 sterilized half-pint jars. Seal with rings and lids, and store in a cool dark place.