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.
Crystal can be produced in mines, captured in a raid, and harvested from a Debris Field. It is stored on your planet and can be spent on items such as ships, defense, technology and buildings.
The amount of crystal produced by a crystal mine per hour can be calculated with this formula:
Its "value" in most universes is about 2 times that of metal and 2/3 times that of deuterium.
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.