Crystals allow special Gold boosts for your Islands.
Place them close to Dragons with the same element to have the ability to boost it's Gold production speed!

You will find different kinds of Crystals , such as:
Electric Crystal, Nature Crystal, Ice Crystal etc.

When placing them down, you will see a circle that needs to be hitting the specific Dragons to give them the boost!
Note that there are a limited amount of these Buildings you can place.