Prepare calcium water; set aside.
In a small bowl, mix sugar and pectin; set aside.
In a medium, heavy-bottomed saucepan, add rose petals, cardamom, and water. Over medium heat, bring to a very low simmer. Gently simmer until most of the pigment of the petals is leached into the water, approximately 15-20 minutes. Do worry if the liquid appears somewhat brown at this point; it will revive with lemon juice addition.
Remove from heat and pour the rose petal cardamom "tea" through a fine mesh sieve into a bowl, squeezing as much liquid out of the petals as possible. Measure the rose infusion and add more water, if necessary, to maintain 4 cups of liquid. Transfer the rose infusion back to the saucepan and add the lemon juice and calcium water.
Return the saucepan to the stove top and bring back to a gentle simmer. Slowly add the sugar/pectin mixture, stirring well to dissolve. Return to a simmer. Remove from heat when perfectly dissolved (you may find that you need to pass this through a sieve again to break up any stubborn sugar/pectin lumps.
TO PROCESS FOR CANNING: Ladle the hot jelly into sterilized jars. Wipe the rims of any drips and place prepared lids and rings on the jars, finger tight. Process in a water bath canner for 10 minutes (for half-pint jars) at a full boil. Remove jars from canner and allow to sit undisturbed for 24 hours.