Clean the rhubarb and dice into small pieces. Measure exactly four cups and place it in a large heavy bottom stock pot. Add the can of crushed pineapple.
Measure the sugar adding ½ cup to a small bowl. Mix the pectin together with ½ cup sugar, mix to combine. Add the pectin mixture to the rhubarb and pineapple in the pan.
Place the pan over high heat. Stir constantly using a wooden spoon. Do not leave the stove, when I say stir constantly, you really need to! It can easily scorch if left unattended.
Bring the mixture to a boil, reduce the heat to medium to medium-high. Boil, stirring constantly for 10 minutes.
After 10 minutes, add the remaining sugar and cinnamon (if using). Return to a boil. Boil exactly one minute. After one minute, remove from heat.
Immediately ladle the jam into clean, sterlized jars. Clean off the rims of the jars after filling. Add the lid and screw on the rings. Process in a hot water bath for 10 minutes. Remove and allow to sit at room temperature until completely cooled and sealed.
This fills 7 half pint jars. Store sealed jars in your pantry. Opened jars or jars that did not seal should be stored in the refrigerator.