That is not an issue that is unique to GOG, though. Yes it's a hassle, and a disappointment, but that would be the same hassle with any other vendor. Did you give them a chance to find another stone or did you write them off completely?
Every vendor has their own definition of eye clean as does every customer, and you have to be VERY clear to every vendor and person you deal with what YOUR definition of eye clean is. But if you don't tell them that up front, they're going to assume you just mean whatever THEIR definition is, and that might not be published anywhere on their site. MOST vendors define eye clean as 9-12" away from face up view only in direct light. My definition of eye clean is can I see it without a loupe, at all, period, in any way, from any angle, in any lighting situation? If so, it's not eye clean. And if I didn't tell them, they're not going to know.