Originally Posted by
niftyshaving
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. The green chlorite mica
may help keep the grain size of the quartz fine and
available as a cutting media.
I do hope someone with a lot of comparison hones gets to try
one of these. The differences between the possible mineral
compositions are large. However, that does not matter
when the proof is how good a hone it makes.
P.S: I do not think the green is kryptonite which would fog
old style film but who knows in this digital world :)