You may have noticed that for each day you log into the game, you will gain a daily reward.
These level up each day, and from day 6 you will gain 1 Gem. Please note that for this you have to log into the each day to level up the rewards, and to collect these.
Note that you only gain the last item on the list, hence on day 1, you only gain the first material, on day 2 only the second material and so on. On the last day and onwards, you will only gain the 1 Gem.
If you forget to log in one day, these will reset back to the beginning.
This is how it looks on day one :
And this is how the daily rewards look like after day six and onwards :