diorite

Quartz is harder than the stone used for them, and diorite can be used to to smooth quartz.

The borer marked 3 on the drawing was of diorite.

Quartz and diorite are harder than granite.

Quartz sand abrasion, hand drills utilizing quartz, diorite smoothing tools and melting with hot coals are all techniques recorded by the Egyptians, and have been reperformed by scientists and engineers with similar results.

By contrast, more mafic rocks, such as tonalite, gabbro and diorite, have 1 to 5 ppm uranium, and limestones and sedimentary rocks usually have equally low amounts.

If they could still do high-quality work: mirror-smooth surfaced, extra-hard diorite, carved with exact bilateral symmetry, you would expect to find some of it done for highest-status people, even if it were no longer squandered on huge monuments.

There is no way I could be convinced that hand-driven bronze tooling was cutting diorite to a precision we would struggle to meet today, carving schists so thin you can see light pass through, absolutely perfect symmetry, and creation of granite stoneware with multivariate surface geometry that we'd be unable to do without a precision-destroying tool change.

The "natron hypothesis" seems to make more sense in Egypt where: Natron and granite powder are just laying around, the blocks are all regular rectangular shape, there are murals that seem to describe the process, and they have large high quality artifacts made from diorite which is the hardest thing around.