grind flat surface on rocks / granite

Hello all

Thanks for all help.
Volunteering at local museum and hobby mines, benefitting from advice
on things here, and hopefully starting work again as a welder
imminently.

Grinding flat surface on granite samples to make visually well
presented samples of geological features - eg. the "contact" between
different types of rock...

Find is true a diamond-plate like for eg. sharpening plane-blades when
on-site produces a beautiful find shiny smooth grind on the rock
samples.

Problem - the removal rate is tiny - need a flat sample to start with
if going to do that.

Advice here is glide tools over the surface and go through finer and
finer grits getting a surface which is polished though not necessarily
machine-flat.  Comments
* this is the voice of experience
* is there really the need for the sample to be machine-flat?

Anyway, I was thinking how it might be possible to produce a flat
surface.

In the steelworks labs. there was the "swing-grinder" which had a
vertical spindle and you swung it back and forth over a sample in the
chuck, lowering it a bit per pass to produce a flat surface to start
going through the finer and finer emery grits with until you could
diamond-polish it to mirror finish.

I thought of base and column of a bench-drill, clamp a collar on the
pillar at height of finishing plane, and have angle-grinder on an arm
you swing back and forth.  Letting the grinder ride-up for light
"cuts", but eventually stopping at the plane dictated by the collar
locked to the pillar (column).

Anyone got a better idea / know how it should actually be done - if at
all?

Regards,
Rich S