Energy Crystals, known also as Brickonium, are green, glowing gem/crystal-like power sources. Energy Crystals first appeared in Rock Raiders, and have appeared at various points since then.
In LEGO Rock Raiders, the L.M.S. Explorer arrived, broken and drained, at Planet U. The Rock Raiders went down to Planet U to mine the Energy Crystals they needed to power their ship back up. While there, they discovered various creatures that used the Energy Crystals as a food source, such as Rock Monsters and Slimy Slugs.
Red Energy Crystals also appeared in the Playstation game, being five times more powerful than normal ones.
Energy Crystals were dubbed Brickonium in the magazines, comics, sets and other sources, though the books and games just called them Energy/Power Crystals.
Energy Crystal patches appeared around the LEGOLAND game. After Professor Voltage traveled to the future, he discovered from Docs just what could be done with these. The player can then build Power Stations on top of these patches.
Energy Crystals once again appeared in LEGO Mars Mission. The Humans here seek out the Crystals to power futuristic technology and ship back to Earth (which has apparently ran out of Energy Crystals), but then Crystaliens appear, who harvest the Energy Crystals for their own needs. The two factions engage each other to secure the Crystals for their respective side. Here, Energy Crystals are gem-shaped instead of crystals.
The Space storyline of LEGO Battles also has Energy Crystals, though these are crystals again. It also introduces bright purple ones with ten times the power of green ones.