Its annoying that he hasnt checked to see if the CP scales with the value of a stone or not. or whether the type has any affect on CP earned. He could be wasting money buying those small ones. By extension, that massive crystal may be worth far more if he used it himself, but im guessing this is a case of the author knowing the small ones are best and not bothering to have the MC check or inform the reader, its kind of a big oops in general unless the MC is just that dumb, i find it hard to believe anyone would be that dumb.