Interesting!

Patina on some old coticules looks like that, but you'd recognize it. There is also a type that oxidizes to this colour (Bart currently has pictures of one on his site I think) but lapping restores the familiar coticule cream. Some Charnley Forests are this olivey greenish brown, but they are much harder and once seen easily identified. 'Brown Eschers' (which I believe to be Vosgiennes) are of a more purple colour. Frankonians are (as far as I can judge from the pictures I've seen so far) decidedly warmer brown, also with a purple note to them, not this more greeny-yellowish hue.

Pyrite and clay remind of Thuringians. Some of them have streaks of brown running through, like the hone to the right in your pictures. I've got a small boxed one with light brown patches; if I can find it, I'll post pictures. Then again, these brown inclusions are without exception very thin. That is to say, until maybe now, I've never heard of or seen one entirely in that colour.

The variety of natural hones is remarkable. But whether to the collector that is a source for infinite joy or the cause of excruciating despair, I still don't know...