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Not very hard... The natural mineral is nahcolite with a Mohs hardness of 2.5 (very soft):
https://en.wikipedia.org/wiki/Nahcolite

Any action you get is chemical.
Etching acids are...

Now that we have all given our caution and warning...
Have fun and if you play with it let us know.

A natural slate and an expensive modern man made hone pose different risks.
To me the largest...

Honing with salt might be manageable on the razor steel but not the hone.

As the hone dries the salt crystallizes and who knows that that does.
I have seen telephone polls feeding power to brine...