Khaydarin Crystals are mysterious and powerful psi-active Xel'naga artifacts that have been found on a select few known worlds, such as Aiur, Shakuras and Bhekar Ro. Generally, Khaydarin Crystal is bright blue in color, and it was used extensively by Xel'naga, and then later, the Protoss.
Khaydarin Crystal possesses incredible psionic powers amounting to levitation. They are partially organic, which justifies their psionic powers. It has been known to be cold to the touch, and Terrans often experience an ache or tingling when touching a crystal. The Protoss feel the tingling as well, but without the pain.
Though the uses of the Khaydarin Crystals vary, only the Xel'naga and their creations (namely the Zerg and Protoss) can forgo their use.
The Xel'naga had used the Crystals extensively for many purposes. They had been used to aid their genetic experiments and had served as computers and psionic databases.
The Khaydarin Crystals had then been used by the Protoss, first being utilized by Khas to restore the Protoss' psionic link and then being fully integrated into the Protoss way of life. Templar, and formerly the Judicator, used amulets set with Khaydarin Crystal to enhance their psionic powers. Like the Xel'naga, the Protoss had also used Khaydarin to store information. Pylons fashioned from huge Khaydarin Crystals are used to extend the Protoss Psionic Matrix and provide power to Protoss structures.
There have only been one instance of the Zerg and Terrans using Khaydarin Crystal. The Zerg had seized a large Khaydarin Crystal formation and collected a cluster of said crystals, which were then absorbed by the Overmind when it had manifested itself on Aiur. In the latter's case, the Terran smuggler, Alan Schezar, had used stolen Protoss technology (including Khaydarin Crystals) to control a Zerg Cerebrate.
"Khaydarin" is a Khalani phrase which means "focuser of the heart." The Crystals were quite possibly given this name because of how they are used to keep calm and focus their psionic powers.
- Argus Crystal