Using a nutcracker or hammer crack open the apricot stones. Remove the kernels and blanch them in boiling water for 1 minute then drain and slip off the skins.
In a large stainless steel or enamel preserving saucepan add the kernels, apricot halves, lemon juice and cup of water. 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 reduced.
Remove the saucepan from the heat then add the warmed sugar and stir until it has completely dissolved. Return the saucepan to the heat then add the butter and stir until melted.
Bring the jam to a full rolling boil and continue to boil for 15 minutes, then test for a set. If setting point has not been reached, boil for 5 minutes more, then test again.
When set point has been reached pour into clean, warm jars and cover immediately with waxed paper discs and cellophane covers.