When playing in Dragon Mania Legends developed by the Gameloft, you will be served with multiple types of currency, in that gems are the premium currency that you have to earn along the game.
In order to earn those gems, you will usually be prompted to purchase them through iap store
However, you can also collect more gems for free without having to spend any real life money when being in the IAP store
Generally, you can get them by completing the quests coming up all throughout the game.
Some of them will earn you gems especially the ones that involve breeding certain types of rare dragons.
Besides, you should complete achievements for free gems that you must check out them by tapping on the trophy icon to appear the list of achievement
Here, you can see what you have already completed and what you need to complete to the next stage for earning more gems.
Still in the achievements, just tap on the icon above the achievements to open up the Dragon Codex.
At this point, you will be assigned to build a collection of a certain group of dragons, and after having that entire collection completed, you will earn free gems as a bonus.
Also, you must check out the limited time events, because they will earn you large amounts of free gems.
Anyway, they will also be one of the most difficult since you will have to compete with the other online players in these events.
Later on, you can then go to the Mystic Cave and the Pirate Ship that you can do after unlocking and exploring for a good reward for free gems.
In addition, be sure to stay alert of random little treasure boxes to come up around the battle map because if you tap them, they will give you one or two gems freely.