The marbles used by the original poster appear to be glass, which is weird because others have tried glass without any luck. I can't believe that glass would stay hot enough to smoke for more than 5 minutes, let alone 4 hours.
In any case, it still could be a great idea. Some people don't realize how significant it would be to completely eliminate coal from the smoking process. Without the coal, it might actually become plausible for people to suggest that hookah is a safer form of smoking.
I poked around on google a bit. It seems that volcanic rock would probably be the way to go, as someone previously said. I was reading about this Maori stone pit oven cooking method called hangi,
in which heated volcanic stones are used to cook food. Apparently they can be heated until they are burning white. But for our purposes, a few questions need to be answered: How long can the stones hold the heat? Can they maintain a temperature consistent enough to sustain a reasonably constant smoking session? Can the heated stones emit any gases that could be inhaled in the smoke? Perhaps it would be necessary to maintain a heat source on top of the stones. Heat coils or burning coals that are shielded from the inhalation airflow could do the trick.
There's one more issue though: I've heard that the reason coals work better than any alternative is that they increase in temperature as you inhale. I don't know how important this really is, but it would seem to me to be fairly significant. I don't think there's any way to achieve this same effect using marbles or stones.