Chevron eight crystal
Stargate control node specifically designed to allow an eight-chevron lock.
The average Stargate cannot dial to other galaxies (i.e. eight chevrons); a special crystal is required to allow the power boost to occur, preventing an overload or other catastrophe. Stargates do not possess additional ports for this crystal. Instead, the device must be manually wired into the base of a dial-home device.
Only one crystal of such design is known to exist, making it extremely valuable. Fortunately, the compact design of the unit allows it to be easily removed and transported to another Stargate if the city of Atlantis fell into the hands of a foe.
Home - Dr. McKay reveals that not any Stargate can dial the Milky Way, and that Atlantis's Stargate possesses the crystal responsible for allowing this to occur.
Before I Sleep - Retreating in defeat, the Ancients leave the chevron eight crystal in Atlantis's D.H.D., allowing them return to Earth following their failed defense of the Pegasus Galaxy.