Wash and cut rhubarb. Wash skin of grapefruit and zest both with a microplane grater. Just the colored skin, not the white underneath. Add the cut rhubarb, grapefruit zest, cardamom and sugar to a large stock pot. Squeeze juice from both grapefruit into the pot also. Stir to mix.

Heat over medium heat until boiling. Stir occasionally and allow to simmer for 30-40 minutes, or until the rhubarb has completely broken down and the mixture is very thick, like preserves.

Remove from heat and allow to cool in pot on the counter for 20-30 minutes.

Spoon preserves into glass jars and store in refrigerator. If storing in freezer for later use, be sure to leave at least an inch of space at top of jar, so your jars don't explode (from expanding preserves) when frozen.

If you know how to can, by all means, can it!