Crystal is the primary resource for electronic components and alloys. It's also vital for research and the construction of buildings, ships, and defensive structures.
In both Uni 2 and Uni 3, similar to the Metal Mine, the crystal production rate is increased (2x) to that of uni 1. Without a Crystal Mine, you'll produce 20 crystal units per hour as a base rate.
Remember, while Crystal is crucial for many aspects of your empire's development, certain items and technologies like Rocket Launchers, Metal Storages, Combustion Drive, Graviton Technology, and Armour Technology do not require it. Efficient management of your Crystal Mine can significantly boost your progress in the game.