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It's tier 3 building in village 

Note 14 says that with depth some ores disappear (depth 46 also don't have aluminium)

(edit most stated problems has been fixed) 

I think that gems and school need some rework because 

1) Even if you have 100 achievements (30 minutes for gems) it takes multiple days to buy upgrades like x1.75 pickaxe (first one) or +5 books per hour (i understand that gems is more like a passive type of upgrade but i don't think that +5 books/hour is deserves 4+ days with 100 achievements)

2) School exams gives you not enough time on higher grades (Literature is hard for someone like me who is not native english speaker)

I think 24 hours of offline progress is a bit too much, maybe make 6 by default and add upgrade for AI  points?