I don't think you're right about that. They judged ore, as the comment above mentions, by the location it came from. They tried standard techniques to process whatever ore they had, and if those techniques led to a bad result, they considered the ore bad.

Andrew Carnegie discovered that certain "bad" ore was that way because its iron content was much higher than usual. Secure in the knowledge that this ore was actually better than "good" ore, he developed the techniques to use it.