Board Thread:General Discussion/@comment-12517932-20130810220511/@comment-10760902-20130811143235

The maximum gems/day a dedicate player can get is: If you win 100% you will get ~7 gems/day => ~200 gems/week. But of course that's just a hyperthetic number.
 * Through Combat World: 4 times x 3 wins / 7 x 2 gems = 3.4 gems/day.
 * Through Stadium or Tournament: 2 times x 2 gems = 4 gem/day.

You can also get extra gems from leveling, Monday Weekly Bonus every week (5 gems is really hot), etc.. :)