I'm getting the feeling that they may have used the name "Carnelian" and "Cornelian" interchangeably in the early years. And maybe to them the name(s) would have applied interchangeably to both of the nontransparent red & white marble types available in the teens and 20's.
That would make sense in that time, I think, because carnelian and cornelian are names for the same kind of stone. IIUC.
Their Dec. 1914 Clarksburg ad says Red Striped Carnelian: http://i119.photobucket.com/albums/o151/modularforms/Akro/1914_12_ClarksburgAkro_Playthings_b.jpg
And Hellmer's batch book gives a recipe for "Copper Carnelians for Marbles" for Akro Agate with a date of 12-1-25.
But of course boxes and other ads we know of from that era say Cornelians.
So maybe it was pretty much the same to them and they only made distinctions between the copper-based and non-copper-based types when it was time to mix up the recipe?