I only read about it something like 10 years ago, when lab-grown started to appear, and they already had the problem of being too pure. Making diamonds with impurities seems to be more costly than making them pure.

What may fool your intuition is that the process is not about refining, it is about growing a crystal. IIRC, the vaporize carbon in a very controlled atmosphere and let it grow.

Natural processes are much more messy and are made in the presence of a lot of other chemicals that need to be artificially introduced in the crystal growing atmosphere for it to be integrated.

That's actually not my understanding. I have heard that most man-made stones are nearly flawless (at least when it comes to occlusions).

Some processes have actually started introducing "fake" occlusions just to make them impossible to distinguish from the natural variety.

Nah, you need all sorts of quality grades for drilling, polishing, etc.

No you don't. Those aren't the hard parts. You use the same high quality, industrial machines to cut and polish synthetic diamonds as you do natural diamonds, and you get the same result. The only quality grades that make sense are color and perfection for natural diamonds, or maybe cutting grades for antique hand-ground diamonds that weren't done well.

For polishing and drilling marble, for example. Not for polishing diamonds. The diamonds are different qualities.