If it is still mined from a known locality, then I'm sure any decently qualified mineralogist or geologist could give us it's correct name. For the seller to buy them from someone who mines them means that someone knows what they are or could furnish the necessary information to help identify them. It would probably be just a name in the form of a mineralogical classification - probably as helpful to us as any other name we choose to call them by, though. Dark Green Honing Slate sounds as good as anything to me!

Regards,
Neil