Large, red crystal embedded within the dial-home device, directly beneath the center "button." The control crystal is essentially the central processing unit for the D.H.D., and is required for its operation.
If the control crystal is removed the Stargate resets itself, and all memory stored within its systems. Upon reinserting the crystal, any stored memory is flushed out, causing a potential overload.
A specially-designed control crystal is also embedded within the Atlantis Stargate's control systems (though it can be removed and installed in a standard Pegasus Galaxy D.H.D.). This crystal is believed to be unique, created by the Ancients as a safeguard to allow Atlantis's gate to encode an 8-chevron address.
48 Hours - With the information provided by Adrian Conrad's Goa'uld, Carter removes the main control crystal on a D.H.D. connected to the S.G.C. Stargate, restoring Teal'c's physical form. In the process, the D.H.D. overloads and is destroyed.
Avenger 2.0 - Carter speculates the primary control crystal might have something to do with allowing her and Felger to reset the Avenger virus.
Home - Dr. McKay removes Atlantis's control crystal and installs it in the D.H.D. on another world, hoping to draw power from the surrounding mist to dial the gate back to Earth.
Vengeance - Michael removes the D.H.D. control crystal on the new Taranian homeworld to prevent anyone from leaving the planet.