Rinse the cherries and drain in a colander. Remove the stones, then add to a pot with 400 ml (approximately 13 ounces) of water. Cook for 10-15 minutes, then puree and place in a strainer lined with cheesecloth. Drain overnight to collect all the juice. The next day, squeeze the fruit until no more juice comes out.
Rinse 6 jars in hot water, then drain.
Squeeze the lemon. Mix the cherry juice and enough water to equal 1 liter (approximately 1 quart) of liquid. Mix with the lemon juice and jam sugar in a saucepan. Bring to a boil stirring often, and cook for 4 minutes. Fill the prepared jars to the brim with the jelly, then seal tightly. Turn upside down for 10 minutes to cool slightly. Turn right side up and cool completely. Store in a cool, dark place.