Thread: Mystery Stone.....any guesses?

Yes, slate can be multicolored: Banded slate whetstones

The clay smell and uniform gray color suggest slate, or rather continue to rule out Arkansas. Do you have a Coticule with a slate back that could maybe serve as reference?

From the wikipedia description of slate: " Occasionally, as in the purple slates of North Wales, ferrous reduction spheres form around iron nuclei, leaving a light green spotted texture. These spheres are sometimes deformed by a subsequent applied stress field to ovoids, which appear as ellipses when viewed on a cleavage plane of the specimen."

..."Fine slate can also be used as a whetstone to hone knives."

So from those pics, it looks like you have a purple-ish/gray stone with green inclusions, almost exactly as is described above for Northern Wales Slate.