I don't think ambition is a requirement, just that ambitious people who actively work toward their ambition are more likely to awaken it. Those who are thrown in the crucible will find out if they have iron.
Trying to scientifically categorize the fruits might be impossible after we received the information the fruits might have been dreamed into existence. In that case it might be enough if people simply believe ravens are just one form of soot and that soot is it's own element.