A crystal mine.

The Crystal Mine requires the same amount of energy as the Metal Mine, however, it produces less due to it's value in technology research. The Crystal Mine is used to obtain crystal, which can be used to upgrade buildings, build ships, research new technologies just like the Metal Mine and the Gas Mine. In addition, mining also takes energy, so you must upgrade your energy production too. Also, the more upgrades the Crystal Mine has, the more it produces just like the Metal Mine.