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 an Energen mine.

Mega Man Zero series
In the Mega Man Zero series Energy Crystals appear as items that serve as currency and are also used to grow Cyber-elves. In Mega Man Zero 3, the Mod Cards 19 and 52 change the appearance of the E-Crystals. As there is an energy crisis during the series, Dr. Ciel created the Ciel System (System CIEL in Japan, Convergent Ignition by Energen Linkage) between Mega Man Zero 2 and 3 to solve the problem. In Mega Man Zero 3, a Giant Elevator was used to bring up a huge cache of Energy Crystals under Dr. Weil's orders. The exact reason Weil needed the energy is never revealed, although it most likely has something to do with Weil controlling the Reploids around the world via Omega and the Dark Elf. Energen is also implied to be the power source for Ragnarok's main weapon, a powerful plasma laser used in Mega Man Zero 4.

Mega Man ZX series
Like the Mega Man Zero series, Energy Crystals appear as items that serve as currency. In the first Mega Man ZX game E-Crystals are also used to repair damaged Biometals.

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.