Using nutcrackers or a hammer, crack open the reserved apricot stones.
Remove the kernels and blanch them in boiling water for 1 minute, then drain and slip off the skins.
Put the apricot halves and kernels, the lemon juice and 1 cup of water in a large stainless-steel or enamel preserving pan and bring slowly to the boil, stirring occasionally.
Reduce the heat and simmer for 15 minutes, or until the fruit is soft and the water has been reduced in volume.
Remove the pan from the heat, add the warmed sugar and stir until it has dissolved completely.
Return the pan to the heat, add the butter and stir until melted.
Bring the jam to a full rolling boil and boil rapidly for 15 minutes, then test for a set.
If setting point has not been reached, boil for 5 minutes more, then test again.
Pour into clean, warm jars and cover immediately with waxed paper discs and cellophane cover. OR process in a boiling water bath.