The Sacred Plasma Crystals are three energy producing gems, similar to the chaos emeralds in the sonic games. Mined on the Shikadi World, they are going to be used to destroy the universe.
One plasma Crystal is given to Keen whenever he defeats a boss creature in Keen GBC. The boss levels are Techno Lab (The Robo Bloog), Shikadi in Space (The Shikadi Overlord) and The Imperial Palace (The Droidican Emperor). Keen returns these to the Omegamatic Warp Drive to save Earth.
It is not known what makes a good Plasma Crystal, nor why they grow to such large sizes in The Plasma Crystal Mines on the Shikadi World, but with the mines littered with gigantic crystals, most of very poor quality but great beauty, the Shikadi would be wise to get into the jewelery trade.