To be honest, I don't know if it's the apple seeds which did the trick with this jam or simply the reduction of the liquid. Either way it worked and the consistency was of great.
The only thing that didn't really work was that the cherries were rather sour and the jam didn't turn out super sweet, but still, like we always do in this house, we ate it :).
juice of one lemon
seeds from two apples
Put them in a big pot with the lemon juice.
Put the apple seeds in a small tea sieb, one that you can easily put in the pot.
Put the tea sieb in the pot and bring to the boil. Reduce to medium heat, but on the lower side (with my cooker it was number 4) and cook for about an hour, while stirring every few minutes. You want the cherries to really reduce and become thick.
While the jam is cooking sterilize two small jars or one bigger. When the jam is ready pour the jam into the jars, close and turn the jars to let the jam cool.