Crystals are one of the 3 currencies used inside LoMM and can be earned and used in a variety of ways.
The main sources to get Crystals is through battles in the World Map and your Crystal Deposits. After winning certain battles they reward you a specific quantity of Gold and Crystals while the Crystal Deposits in your Base generate Crystals permanently, so be sure to check them often and collect the Crystals before they reach their max capacity!
You can check the amount of Crystals you currently have at the top right corner of the screen when you are at your Base.
After you collect the Crystals you can use it in a variety of ways, the main one is to upgrade your Heroes, Spells and forge equipment. Crystals are crucial for the development of your game because they are necessary for all your team upgrades and are responsible for everything that makes your Heroes stronger.
Crystal Deposits - These are your Crystal mines, the more you upgrade them, the more Crystals they generate for you, but remember whenever the deposits reach their max capacity they cannot produce more Crystals until you collect what has already been produced! You can collect by tapping the deposits whenever the Crystal icon is visible.
Crystal Storehouses - This is where your Crystals are stored after been collected, which means you have a limited capacity of how many Crystals you can store before spending it. Upgrading your Storehouses increases the capacity that can be stored in each one of them.