Inspect four 1/2-pint jars for cracks and defects. Sterilize by immersing in simmering water. Wash new lids and rings in warm soapy water.

Remove stems and blossom ends from crab apples. Cut fruit into quarters and place into a large stainless steel pot. Add cinnamon stick and enough water to just see it through the fruit. Bring to a boil over medium-high heat, then simmer until fruit is soft (10-15 minutes).

Discard cinnamon stick and strain the fruit through 2 or 3 layers of cheesecloth. Collect at least 4 cups of juice and discard the pulp.

Return juice to the pot. Simmer for 10 minutes, skimming foam. Stir in sugar, then boil until the mixture reaches 220-222°F (104-106°C) (about 2-5 minutes). Remove from heat.

Pour jelly into hot, sterilized jars, leaving 1/4 inch of headspace. Remove air bubbles, clean the rims, and seal jars tightly with lids and rings.

Place jars into a stockpot with a rack at the bottom. Add boiling water to cover jars by 1 inch. Boil jars for 5 minutes.