I've no idea on the marble. Could it simply be a case of preventing burning/sticking? The boiling of the jam would presumably have the marble rolling around a lot in a similar action tho using a marble or small stones in a jar with water in it to shake it up and clean the glass inside. Usually done for stubborn stains where you can't get a scrubber to.
Ok, a quick search found a single reference (only looked on page 1 of results though).
Mrs. Cornish's Easy Apricot Jam
