of all the sulphides that Ive torched, Ive never seen one blow up like he had in that one photo. No ideas.
The advantage of melting vs grinding:
With melting you will retain more volume and also it will be annealed so you know it should hold up. The other thing with melting is, sometimes you have to tweezer off little white spots on the surface from dirt and crap that got down into the chipped areas.
with grinding, you loose more volume and there is no stress relief in the glass, so what ever stress was there, still is.
Nice looking marble.