The app was doing good but it seems to contain some nefarious data collection on the Claim free chest ads, as sometimes the "ad" you get from it is Duolingo closing and another one of your currently open apps popping up for a brief moment before closing again. Open up Duolingo again and voila, gems! Trying it with no other apps open results in Duolingo closing, no other app flashing on screen and no gems, for 3 shots in a row until it gave up and showed me an insurance ad.