Board Thread:Plants vs. Zombies 2: It's About Time/@comment-11409759-20150118164356/@comment-7234182-20150321065305

From my experience, you get a gem 95% of the time when you play a normal level for the first time in a few hours or so. Then it will probably take you 3 or 4 tries before you get another gem from normal levels. After that, you don't get any gems (or it's very, very rare). I haven't quite figured it out yet, but it seems like there's some sort of timer that resets after a certain amount of time.

The best bet is to play a lot of endless as those give you plenty of gems with no apparent limits. And play Pinata Parties when you can; you might get lucky from time to time. The "tricks" to pick a certain pinata are most certainly bogus.