I'm certain it isn't a coticule. It's too hard, there are no layers visible on the flanks as there are on hard green coticules, and I can find no trace of spessartine.

It does have the particular Charnley Forest mossy green complexion. There are patches with diffuse purplish hints, and there is a purple vein too (difficult to see in the pictures, but the leftmost queer one).

I read about non-Whittle Hill stones having (hard) inclusions and being stingy, but I don't know what form the inclusions take. It's the first time I see inclusions of this kind in a CF-like stone: as if crevices have been filled with fine hour-glass sand of matching colours and an occasional small black pebble.

In want of alternatives, my best guess would still be it's a CF, but hone afficionado hi_bud_gl had some interesting contrariant observations. CF, he reasons, is a very hard and very slow-cutting hone. Therefore most of them were cut oblong in lenghts above 8", usually even 10"-13", otherwise honing would have been far too weary. A CF of only 5", like this one, wouldn't make sense. Therefore it most likely is something else.

- but what?