Black granite is a lot heavier (more dense)and harder than the standard Barrie Grey. This sarcophagus would have been a monster job for whoever fashioned it. The black granite I work with rings with a high clear tone when struck with a rubber mallet.
We can hardly fashion such objects with the technology of our day. That granite box was hewn from a single block of stone.
How did they lift it? How did they cut the large flat surfaces, and smooth them to microscopic tolerances? How did they hollow it out and square off all of the inside corners?
I think that object was created by someone far older than the Egyptians or the Greeks. Artifacts such as this, are pointing to the existence of an advanced culture that has been completely lost to history.