It means that you can't tell whether it's a "higher grade of Type A" or a "lower grade of type B", because both shine the same, and you can't tell the underlyin

It means that you can't tell whether it's a "higher grade of Type A" or a "lower grade of type B", because both shine the same, and you can't tell the underlying purity of either nor the type of the stone you are looking at. Only if you know either the grade or the type (using high-tech instruments) can you infer the other from the observed shine.