Pectin is a naturally occurring substance that makes jellies gel and gives jams their yummy spreadable consistency. Fruits that are low in pectin usually need to be combined with high-pectin fruits in order to get a good gel. Another option is to add commercial or homemade pectin.
Use the lists below to find out which fruits are high in pectin or low in pectin. The point to keep in mind while you're looking up the fruit that you want to preserve is that if it's on the low-pectin list you will either need to add pectin or combine it with a high-pectin fruit.
Strawberries and red currants are a classic example of combining low and high pectin fruits in preserves (it helps that both are in season at the same time).
It's also a good idea to remember that fully ripe fruit, while delicious, is lower in pectin than slightly under-ripe fruit.
High Pectin Fruits
Low Pectin Fruits