Some tips to hopefully reduce confusion:
(1) When Ore Carts get to 100% reduction, that is a rounding error. Buying more still makes refining faster
(2) Don't forget to buy smelters!
(3) Charm upgrades are not permanent; they only last until your next reset. That's ok, though, because you should have a lot more charms and not care about the cost of the previous charm upgrades.
(4) The total number of charms you have earned is based on the total amount of money you have earned.
The problem I have with this game is how the various mines are not balanced at all. From the moment you unlock diamond mine until the moment you get 3000 copper mines (undecillions of charms!), diamond mine is the best. It would be nice if other mines got a chance to shine.
Thanks for the great comment! I agree that the other mines need to be more useful late game. We are investigating some fun ideas, like relics and crafting with other elements than iron to make all the mines more interesting!
TheLowerLight is wrong about the iron mine eventually pulling away. You can always speed up the smelter and refining time enough to compensate. My level 200 iron mine requires 170 levels in "Ore Carts," which I can get about 20 minutes after resetting.
ZzAlexzZ - the developer has been pretty clear that this game is early in its development, and there is a lot planned for later. Maybe someone like you should come back in 3 months.
Some Suggestions: 1. A Stats Screen. 2. A different multiplier bonus curve - currently I can get to over 60k additional multiplier in 15 minutes, but playing for 24 more hours only gets me to 90k. 3. List total value of the game instead of "current" value... I want to know how much I'll make for finishing the game instead of how much value I've already created (the % completed takes care of that). Relatedly, 4. Indicate how much "Interval Income" the current game will add. 5. Retune the click value curve; clicks are useless pretty quickly.
There seems to be a problem with the mutiplier gain on reset. I've reset a few times, and now after about 15 minutes of play, I get 70k multiplier bonus... then 4 hours later, it's only at 88k. Something is not right. (And yes, I realize that you only get bonus for cash in hand... I had about 10^8 times as much cash on hand for the 88k bonus than for the 70k bonus)
inventory management knocks off one star, and the VERY poor tutorial (I died before leaving the city the first time because I couldn't find the exit!) knock off another. 3/5 for what is nearly an excellent Harvest Moon variant. A sequel - or moderate update - could easily be 5/5
 "Treasure Chamber! Idle Mining Adventure")
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Thanks for the great comment! I agree that the other mines need to be more useful late game. We are investigating some fun ideas, like relics and crafting with other elements than iron to make all the mines more interesting!