My ideas on this....

As soon as the block encounters the atmosphere a shock will form on its outer surface. Within this thin layer of air temperatures are extremely high - 1000's of degrees.

A rock is made of a variety of solid oxides, or ceramics, which have a very high melting point. It is more likely that the surface will vaporize before large melted blobs come away.

A metal block will conduct heat and probably melt. Air resistance will ripple across the surface tearing away larger blobs.

So I am guessing that your idea will not work. Besides we can mine more nickel-iron on Earth than we could ever get in space, and for a fraction of the cost. On the other hand if we intended to keep the refined material in orbit and build something out of it, then it might be more valuable. For example it would cost $10,000 per pound to launch it. So I suspect that a 1,000,000 tonnes of titanium alloy for example that is already in orbit might be worth something.