OK so after a lot of practical research on how to improve my use of the prospecting tool, I finnally begin to understand how the tool actually works (if you have precise data it would be huge though, like what zone does it search, how many blocks, what is the precise meaning of "rare", "poor" etc...).
So I set myself to a proper, clean session of tryhard prospection. After too hours of not being able to get more than "very poor" on a hematite concentration, I began to get crazy, and finally I turn my solo game in creativ mode and go mad pickaxing every corner of the floor. Only to find out that my hematite was literraly right 30 blocks bellow my main researching point.
So I go and prospect the very touching block to this huge hematite gisement, and... "very poor". I try to stay cool but I think my mind is forever broken.
Is this a bug? Tell me this is a bug, please.