Sadly, I believe you are probably right. It does beg the question, though, how pure does a metal have to be to be Allomantically/Feruchemically/Hemalurgically viable? Iron, for example is usually found naturally as iron oxide, and aluminium oxidises near instantly on contact with air. Even modern metals have impurities, as evidenced by the fact that iron and steel spark will spark when being forged with a hammer. The sparks are slag, little specks of rock, trapped within the material.