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If you use an Android device, you can install the Google Opinion Rewards app from the Play Store. This app will send you the occasional marketing survey, and you'll be rewarded Google Play credit for completing them. You can then spend this free Play Store credit on Clash of Clans Gems. Most surveys only take a few seconds to complete, and award between $0.10 and $1.00. This app is an official app developed by Google, and is safe to download and use. The Google Opinion Rewards app is not available for iOS devices. If you aren't already signed in on your device with a Google account, you'll be prompted to sign in. Google accounts are free to create. You won't get many surveys unless you turn on location services. This is because many surveys are based on locations that you have recently visited. Open the Settings app on your device and select "Location." Ensure that "Location" is toggled on at the top of the screen. Allow location access if prompted when opening the Google Opinion Rewards app. You may not have any surveys available immediately when you first start the app, but they should eventually start to appear. You'll get more surveys by visiting businesses and traveling a lot. The answers you give do not affect the value of the reward you will receive. You'll receive a notification on your device when a new survey has become available. It may take a little while, but you'll likely amass a nice amount of Play Credit before you know it. Once you have enough to afford some Gems, open the Gem store in the game and buy a pack you can afford. Make sure your Google Play balance is selected as the payment method during the checkout process.

Summary:
Download the Google Opinion Rewards app. Sign into the app with your Google account. Enable location services on your Android device. Complete any available surveys. Continue answering surveys until you've accumulated enough Play Store credit to purchase Gems.