Below you can find a list of coupon codes which grant free rubies:

DEEPFENSE
GCSTEWGAMING1
GCMANGOSEVEN1
GCKINGSOYER
GCKIMSILJANG1
GCHIICHANNEL1
GCLANCAT
GCTKGODKNEE

In order to active them you need to go to the options ingame.
Go to the "Other" tab and click "enter coupon".
A link in the browser opens where you need to enter your player tag.
You can find it in the first ingame options tab.
After that just enter the coupon codes and active them one after another.

If you know any other working codes, let us know in the comment section down below!
If this helped you I will be thankful if you vote up this guide or any of my reviews, thanks!