I'd listen to these folks. I can't identify hones by eye for crap, but that looks enough like an escher to me I'd take their word for it and eschers are one of the few naturals I know of that warn against use of oil (The only other stone I know of that looks similar to an escher to me are certain shards of black arkansas and those prefer oil).
Hell, list it in classifieds for 75% of what it's worth if it is a thuringian as an "unknown stone" and see if someone buys it. ;)