Wash and chop the rhubarb into small pieces, about 1/2 to 1 inch. You do not need to peel the rhubarb.

In a large, heavy-bottomed pot or Dutch oven, combine the chopped rhubarb, undrained crushed pineapple, and granulated sugar. Stir well.

Let the mixture sit for at least 30 minutes (or up to a few hours) to allow the sugar to draw out moisture from the rhubarb. This helps prevent scorching.

Place the pot over medium-high heat. Bring the mixture to a rolling boil, stirring frequently to prevent sticking. Once boiling, add the butter (if using) to help reduce foam.

Reduce the heat to medium-low and continue to boil gently, stirring occasionally, for 45-60 minutes, or until the jam thickens. To test for doneness, place a small amount on a chilled plate, wait a minute, then push it with your finger. If it wrinkles, it's ready. Skim off any excess foam with a spoon.

If canning: Ladle the hot jam into sterilized hot jars, leaving 1/4 inch headspace. Wipe rims clean, center lids, and apply bands until fingertip-tight. Process in a boiling water canner for 10 minutes. Remove jars and let cool completely on a wire rack for 12-24 hours. Check seals before storing.