Pretty simple. First you pick two rocks that have nearly the right shape (its obvious they did that, because each joint is clearly custom). You put some dust on one surface. Take the mating surface, and rub it a bit on that dust. That will reveal the high spots. Take a smaller rock and bang on the high spots to grind them down a bit. Repeat the process until no more high spots.
It's not dissimilar to how telescope mirrors are ground to perfection, and how to make a perfectly flat steel surface.
Doesn't explain the entire lack of tool marks, though. Or the lack of embedded shells.
I am curious about the perfectly flat, right-angled interiors of the 50+-ton (always empty!) basalt boxes in Egyptian mausoleums. We have a plausible story about cutting the outsides using copper saw blades with gems embedded in the edge, but no similar one for the insides, particularly the inside corners.
We also need to understand why the tech for working the biggest rocks was lost before the Inka took up building. The late work always uses much smaller rocks, and is relatively crude.
> Doesn't explain the entire lack of tool marks, though.
Because they didn't use tools. I've watched telescope mirrors ground to incredible precision by hand, no tools.
> Or the lack of embedded shells.
Use rocks without embedded shells.
> inside corners
Use drills, clean up with files or blocking. Remember the Pharaohs had essentially infinite resources to do that for one box. He would have used the best masons, paid them well, and gave them the time they needed.
> why the tech for working the biggest rocks was lost
We can't know why, but the most plausible explanation is one that accounts for lost technology everywhere. People didn't have writing or didn't bother to write it down or lost the books.
>> Doesn't explain the entire lack of tool marks, though.
> Because they didn't use tools. I've watched telescope mirrors ground to incredible precision by hand, no tools.
Good one. That doesn't work on non-convex polygonal shapes. Notably, the faces are not just identically curved, as one would get by interfering faces. They are both flat.
>> Or the lack of embedded shells.
> Use rocks without embedded shells.
The rock faces in the quarries have embedded shells. Only the cut faces of rocks from those quarries lack them.
>> inside corners
> Use drills, clean up with files or blocking. Remember the Pharaohs had essentially infinite resources to do that for one box. He would have used the best masons, paid them well, and gave them the time they needed.
Infinite labor doesn't help in a box. Only one or two masons would fit. We know what their rock drills were like; they had a minimum radius measured in inches. And, again, no tool marks.
You don't get to define out the unknown by inventing excuses to ignore it; that is the same as inventing aliens. It takes actual work.
It's not dissimilar to how telescope mirrors are ground to perfection, and how to make a perfectly flat steel surface.