Crystals refer to crystal tiles. These are randomly upgraded letters which are different color than the usual ones and always give some kind of boost. For instance, bronze one gives you shield, red one allows you to steal enemy's health for one turn and so on.
If you're looking for a better chance of having them two at once, you can buy an upgrade at the store that gives a few more percents of chance of getting the crystal tile as well as a book later in the game that as well rises the percentage of getting it.