If I'm in the mine and quit the game by pressing my iPhone button, mined cristals are not saved and are lost (I lost 10000 like that the other night -- went to bed thinking about the shiny blueprint I would buy the next day, and in the morning, pfuit! no more cristals).
I'm playing The Beginning.
I also noticed that if you exit directly from the store, recent purchases are not recorded.
So, I'm trying to remember to "leave mine" and "leave store" before leaving the game, but in some situations it's not really practical (incoming phone call... ticket controler on the bus...)