Marble spheres are primarily used on the face and neck, the hands and feet, arms and legs, and the back. When used cold they are particularly good for reducing facial inflammation, for treating sinusitis and for working on the trapezius muscles and the neck. When used hot they are great for massaging the hands and feet, the trapezius muscles and neck, and parallel to the spine.
They can be used both cold and hot. Marble actually holds heat slightly longer than basalt, but generally basalt is used for hot stone work as it has more grippiness with the connective tissue when used as working stones (stones that are used to massage) but there's no reason not to use marble stones as hot placement stones, though you need to avoid using them in areas where they are more likely to slip off because of their smoothness.
You can find more information on the use of marble spheres in cold and hot stone massage by searching for "marble spheres massage" on youtube.
When using marble spheres for hot stone massage you should keep the stones moving (especially when used on the face) if they are hot. You can stop moving a warm stone but not a hot one.