Gem is the main item of Coin Dragon Master.

You can get gems like the below.
1. Shop
2. Daily Shop
3. When you increase Level of Account
4. Main Quest
5. Advertising Reward

And you can use 'Gem' like the below.
1. Purchase 'Coin' on Shop
2. Purchase 'Spin' on Shop
3. Purchase 'Dragon Chest' on Shop
4. Purchase 'Dragon Token' on Daily Shop