Combine rhubarb, raspberries and white grape juice in a large pot. Cover and cook over medium heat, stirring frequently, until rhubarb is tender (approximately 15 minutes.)
Once rhubarb is tender, add sugar and lemon juice. Stir to combine. Bring to a boil, then reduce to a simmer. Simmer until reduced to desired consistency, stirring occasionally. For me, this takes 45 minutes.
Skim foam as needed. When jam is nearly done, stir in 1 tsp butter. This will help keep the foam down.
Submerge rings and lids in a bowl of very hot water and set aside. DO NOT BOIL LIDS.
Ladle jam into hot, sterilized jars leaving 1/2-1" headspace. Run a small spatula around the inside of each jar to remove air bubbles. Carefully and thoroughly wipe the rims of the jar clean. Place the lids and then rings, tightening until fingertip tight.
Process jam in a boiling water bath for approximately five minutes. Remove jars to a clean towel away from direct sunlight. Let rest for 12-24 hours.
Test lids for adhesion. If lid did not take, refrigerate or reprocess jam. Otherwise, store canned jam in a dark cabinet. Will keep for one year.
Try serving the jam on top of partially melted brie cheese. Absolutely delicious!