well maybe some will say that is pure luck or something but i can say that is tested cuz i have 12 miners.All day i put them in row and they mine at the same place 24/24...after 1 or 2.....they dont mine any gem!(be 4 i had 1-2 super gem a day) they even had ore.So i place them in the area 1 per screen and the first day i've done this i had a super nix and ora were mine a lot faster....
so the facts is: NO RANDOM!!! if there is too many ppl in the same screen it will ****.I hope this will help a bit
have a nice day ^^
a water tao from Statue Of Zeus