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Yes it is very difficult to tell just by looking.
And as a long time field geologist I know
that quartzite, chert and flint can come in all
shades of color, texture and grain size.

It is a...

If I had to make an educated guess from the picture
I would call this a "Chlorite rich quartzite". If it is, it
would hone a lot like a black Arkansas or perhaps a
translucent Arkansas hone. ...

I suspect the "beryl" is a reference to the color not the
mineralogy of the material. If it is a clean solid anything
with a nice smooth grain it may be a fine polisher no matter
what a...