The crystal is a fist-sized Forerunner artifact when originally seen by Dr. Halsey and the SPARTANS in the underground facility on Reach. It was floating upright and the area around it was distorted by gravity. The crystal reacted with a burst of neutronic radiation at Halsey's touch. When exposed to a Covenant gravity beam the crystal nullified it, reshaping itself in the process.
When the Ascendant Justice jumped into Slipspace the crystal again rearranged itself, emitting radiation during the entire jump, and caused gravitational distortions. The location of the ship at the end of this jump was somewhere near the Eridanus system, a distance that from Reach should have taken several days instead of a few minutes. Comparing timestamps from several missions Dr. Halsey discovered a 3 week average difference. Her conclusion was that the crystal bent both space and time so that it could be removed from Reach by Cortana and the Master Chief. Halsey later gave the crystal to Corporal Locklear, who covered it with C-7 explosive and blew it up to avoid another Slipspace radiation spike.
Three shards of the crystal were recovered by the Covenant, which were presented to one of the Prophets by Tartarus in a small container.