The Irk Crystal was an object of great power in the Invader Dib storyline. This crystal originally belonged to ZIM, who ruled Irk during the Eons of Peace. By the power of the Irk Crystal, the people of the Earth were able to enjoy long, peaceful lives. The Irken Republic watched over the planet Earth and the Universe in this time of peace.
Exposure to the Irk Crystal renders all of Earth's inhabitants functionally immortal. Anyone not born on Irk becomes essentially locked at their current physical age, as shown by Gazlene, her brother Dib, their father, and even their court , who all appear as if aged in their early twenties.
ZIM mainly uses the Irk Crystal as a healing and immortality tool. It could, however, be used as an offensive weapon, and was used as such to destroy various intruders to the Republic. It was one of the sources of his power as the Tallest. The ability to use the Crystal's power, however, appeared to depend on the user, as it cost ZIM a bit of his energy more than once to do so; the times it did not, it was because someone else had combined their powers with its.
To prove to his future empress that he could give her power beyond imaginings if she became his consort and sat beside him in power, he broke the Crystal in half; this caused it to regenerate into two crystals of the same shape. Gazlene then charged her crystal with some of her inherent dark energy, which in turn charged and empowered ZIM's own crystal.