The description above still applies to the parts you melt.

The point is that when you heat a rock and then cool it, in different circumstances from those in which it was originally formed, you're making a different kind of rock, with different physical properties. Every atmospheric oxygen and moisture are an issue. And minerals haven't been refined the way industrial metals have, so you don't even really know exactly what you're heating up. The chance of the molten part staying stuck to the part that didn't melt is pretty slim.