Crystal is one of the 3 main resources in the game. It is advisable to upgrade the crystal mine daily, because a constant supply of crystal is required for research and energy development, being required in the most quantities for researches. It is used to create electronic circuits and form alloys.
The amount of crystal produced by a crystal mine per hour can be calculated with this formula:
When making ships however, metal is more important than crystal, and a good metal output is recommended if one is focusing on making ships. That said, a crystal shortage is one of the most common causes of production cessation.