I feel that the gems are way to far and in between. They should be a little bit easier to get I have never been able to get extra balls for what ever price in gems it is. I never had the amount needed for auto daub or the 20 to play more cards in the tournament either. Sometimes I have to use them up to replace keys when the keys run out. It seems that the game use to let you have a chance to buy more keys instead of using the gems. But if you don't have the coins for keys then it went to the gems to open chest.
Just my 2 cent. All the stuff you gather up during play should be used to trade for anything tickets for gems, keys for tickets power ups for keys, gems and tickets do this with all the stuff a player uses, so on and so on.
You can buy keys after a game. You just have to do it before you open any chests. There should be a tiny window that tells you how many keys you have, and gives you the + to buy. Once you open a chest, then that goes away, so if you run out of keys after you've opened one, you have to spend if you want to continue.
I don't know what level you are, but I've found that sometimes, the best thing to do is suck it up and not open the chests. Especially in the tournament. Yeah, you lose out on some random rewards, but you save your keys for when you actually need to open chests (like when you're hunting for collectibles).