Energy Crystal

Energy Crystal, known as Energen Crystal (エネルゲン水晶) in Japan, E-Crystal (Eクリスタル) and EC for short, is an energy resource from the Mega Man series used for Reploids.

Mega Man X series
Energy Crystal is mentioned in some games from the series. In Mega Man X2, Crystal Snail took over an Energen Crystal mine. Tunnel Rhino from Mega Man X3 originally worked in an Energen Crystal mine before becoming a Maverick. In Mega Man X: Command Mission, the Gimialla Mine was a Energen mine.

Mega Man Zero and ZX series
In the Mega Man Zero and Mega Man ZX series, Energy Crystals appear as items, which also serve as currency. In the Mega Man Zero games they are also used to grow Cyber-elves, and in the first Mega Man ZX game they are used to repair Biometals. In Mega Man Zero 3, the Mod Cards 19 and 52 change the appearance of the E-Crystals. Also, an elevator was also used to bring up a huge cache of Energy Crystals under Weil's orders. The exact reason is never revealed, although it most likely has something to do with Weil controlling the Reploids via Omega and the Dark Elf.

Other media
A Reploid from the Resistance gives an Energy Crystal for Zero in the Rockman Zero manga.

Trivia

 * The Energy Tank was misnamed "energy crystal" in the North American Mega Man 2 instruction manual.