Sterilize 5-7 pint jars (wash thoroughly then heat for at least 10 minutes in a 250F oven, keeping warm until needed, and prepare seals in boiling water and keep ready.
Stir together in a non-reactive large saucepan (really big so there is lots of room for expansion!)
1litre/4 cups blueberries and raspberries, slightly mashed
1 packet Certo crystals
30ml/2 tbsp lemon juice
Measure out in a bowl:
1.25 litre/5 cups granulated sugar
Bring fruit and Certo mixture to boil over high heat, stirring constantly. Add sugar, return to a hard rolling boil that can’t be stirred down. Boil hard for one minute.
Remove from heat, stirring and skimming off any foam for 5 minutes.
Fill sterilized jars to within 1/2 cm or 1/4 inch from the top. Seal jars. Snap lids will pop when the seal is complete.