It has the heavy chamfers of the French hones
That definitely looks like a French stone chamfer though
Anybody fancy having a guess now it is all cleaned up...? French stones (and purple stones generally too) are a bit of a blind spot for me in terms of distinguishing features, apart from I know they can all look pretty similar.
I can't see any green in it, and it's not the finest stone in the world - I don't think it would finish a razor. These pics are wet.
And while we're at it, here's a smaller slate hone with chamfered sides I just cleaned up. The last pic shows it on the right next to a Thuri on the left. Visually they're extremely similar, but I don't think it is a Thuri - it's not quite as fine, but could still probably be used to finish fairly well.