 One might think that  15a  can be justified only empirically, but gemologists seem to think that it is conceptually necessary that rubies are red, that is, that the meaning of “ruby” requires that rubies be red. So given how gemologists use “ruby”,  15a  is knowable a priori. The notion of a ruby seems to be a hybrid notion: its color is knowable a priori but its material essence as given in  15bi  is knowable only empirically.