Homemade Blackberry Plum Jam - cooking recipe

Ingredients

3 cups plums, chopped and pitted (leave the skins on)
3 cups blackberries, crushed (do not puree, crush with potato masher)
6 cups sugar (add less if desired)
1 (1 3/4 ounce) package dry pectin (Sure Jell works well too)

Preparation

Prepare the boiling water canner before starting to cook your jam. (Fill the canner/stockpot at least half full of enough clean, warm water so the level will be at least 1\" over the top of the filled jars once they're added).
Wash, pit and chop plums w/ skins.
Wash and crush blackberries w/ potato masher.
Place plums and crushed blackberries in large pot.
Add pectin a little at a time to plums and berries, stirring constantly.
Heat on high stirring constantly until mixture comes to a FULL boil.
Add sugar ALL at once, stirring.
Bring mixture back to a full hard boil, stirring constantly.
Use potato masher to carefully further crush plums and berries in pot.
Boil for 5 minutes.
Skim off foam, if present.
Place in sterilized, hot jars, leaving 1/4\" headspace.
Wipe edges of jars with clean cloth, place lids and screw on bands fingertip-tight.
Process in a boiling water bath for ten minutes at sea level (over 1000 feet, add one minute of processing for each 1000 feet in altitude).
Remove from water and place jars on a cloth-protected counter or cooling rack to cool undisturbed for 24 hours.
Remove rings from sealed jars and store; put any unsealed jars in the refrigerator and use first.