Argus Crystal is a rare and mysterious mineral known for its psionic properties. It similar to the Khaydarin Crystals used by the Protoss and Xel'naga, and it is very possible that they are Xel'naga in origin. Not to be mistaken with the Argus Stone.
Argus Crystal is to the Dark Templar as Khaydarin is to the Khalai. While not actively used in the Protoss Warp Matrix, its main use is to focus the Void energies used by the Dark Templar.
Dark Archons wear talismans set with Argus to magnify their terrible psionic attacks, and Corsairs can be fitted with Argus Jewels which make their Disruption Webs more energy efficient.
Before the official release of Starcraft II, there was a Dark Templar version of the Protoss Pylon which was later scrapped. It can be assumed that the "Dark Pylons" used Argus cores instead of Khaydarin.
Argus was the hundred-eyed servant of Zeus's wife, Hera, who had been sent by the latter to watch over Io, who had been transformed into a cow. He was then bored to sleep by Hermes on Zeus's orders, and then had his eyes sealed shut by the former. His eyes were then put onto the tail of the peacock by Hera.