We have old, really old, fruit trees including plums (purple, red, yellow and gold with blush). The fruit are small and tart but delicious. Removing the pits, however, is not fun. This method worked very well.

Spread plums on cookie sheets (with rims). Roast at 400 degrees for 15 minutes. The skins for the most part pop up and sit on top of the plums like little paper hats. They are easily removed. The skins from the red and purple plums, however, roasted to a gorgeous ruby color and I added some of them back to the mix. When cool the pits are easily mooshed out of the fruit.

For jam I added the juice left in the pan back into the fruit mix. I also did batches where I used the fruit to make plum crumb pies and saved the juice from the pan to make hot pepper jelly.