In a Dutch oven, combine rhubarb, sugar, pineapple, and water. Bring to a boil. Reduce heat; simmer, uncovered, 18-22 minutes or until rhubarb is broken down, stirring occasionally. Add gelatin; cook and stir until gelatin is dissolved.

Remove from heat; skim off foam. Ladle hot mixture into hot half-pint jars, leaving 1/4-in. headspace. Remove air bubbles and adjust headspace, if necessary, by adding hot mixture. Wipe rims. Center lids on jars; screw on bands until fingertip tight.

Place jars into canner with simmering water, ensuring that they are completely covered. Bring to a boil; process for 10 minutes. Remove jars and cool. Yield: 7 half-pints.