I would say its a slate...i own several slates like this. Actually its difficult to point out the origin of any of those.
Mottled slate appears in many different regions of the world...
Iam actually trying to get some analyses done from a geologist. To get thin cuts of some stones. This would be the only way to try to compare those samples with others beeing already done.
The coloration comes from a Redox reaction.
Red comes from Hematite, green comes from Chlinochlor....
I also talked about "Vosgian Stones" to a certain time for my stones. I have to admit today this Term cant be proofed in detail....
Here are some pictures:
https://www.flickr.com/photos/razorl...57650718803678
Here is a sample of a mottled Vermont Slate:
http://up.picr.de/25935659vs.jpg