[Radim]

Well yes, that's my point – the process is not trivial, with surprising technical details.

For a more in-depth take on grooves – at least more in-depth relative to "concentric circles" or Lehner's "wet sand" video) check out my link above, https://antropogenez.ru/drilling/. Specifically on Petrie's testimony they offer this:

> Of course, Petrie’s Core#7 does not bear any regular helices or a thread cut in granite with a fixed jewel point with a pitch of 2.0 mm, as it has been described by him. There is only a series of grooves, the formation mechanism of which is described above in detail. Their pitch, being very irregular, is not related to the advance movement of the tool cutting edge.

Most importantly, they ran actual experiments on actual stones.

And their theory for those grooves is a sort of emergent property of the accumulated effect of corundum grains falling into the same crest/trough pattern along the tube wall while drilling downward, leading to the observed series of (irregular) cut spiral grooves.