- last post: 01.01.0001 12:00 AM PDT
The crystal you refer to is the one that bent space and time to allow the MC and Cortana to arrive back at Reach in time to save Dr. Halsey, the remaining Spartans, and the crystal itself from being taken by the Covenant.
Also, when the Covenant ship made a slipspace jump inside the Earth's atmosphere, it only destroyed a portion of the surrounding area. It's not like the entire Earth was swallowed up and destroyed by that shockwave in the CGI.