The top upgrade card says that mining has an 8% chance of earning gems, but after 793 mining actions I had only 13 gems. If the chance is actually 8% this is basically impossible (p ~ 10 ^ -15).
I haven't kept careful track, but I'm not seeing gems anything like 1/12 of mining actions either. However, perhaps it's 8% of rocks. (I can't recall ever getting two gems out of the same rock.)
Agreed, I have been wondering about this for a while now. I get gems MUCH more rarely, even with all the gem upgrade cards, than I used to... I didn't have the math skills to prove it though, nicely done :-)
Hey folks- your chances of getting the gems will decrease the more you get in a game day; we'll try to make that more apparent or give some indication about this when you are mining.