Data crystal chip

Data crystal chips are a form of file storage device used by the UNSC. These data crystal chips are used for transferring "hard" data, meaning data that cannot be transferred through a transmission because of the data's sensitive or complex nature. Examples of such data include AI data, navigation data, weapon technological charts, etc.

Overview
The data crystal chip can be utilized for many things. Notably, they are used for storing the Riemann Matrix for an artificial intelligence construct, like Cortana or Deja. They can also carry star charts, maps, and other navigational data. These chips must be destroyed if contact is ever made with the Covenant, as per the Cole Protocol. This did not happen aboard the  during the Battle of Reach, so Blue Team were sent to destroy its data crystal chip. In general, they are used to carry sensitive data, such as intelligence. Dr. Catherine Halsey had compiled two of these with all known information on the Flood (one had the complete information on it, including how Boren's Syndrome affects the Flood's ability to assimilate an individual into the Flood super-organism. The other had this data omitted, in that if the complete report were to be found, Sergeant Major Avery J. Johnson could be killed and autopsied by ONI, because of his being afflicted with the disease).

Trivia

 * The data crystal chip was seen extensively throughout Halo: Combat Evolved, as the Master Chief uses a data port on the back of his helmet ferry Cortana between his armor and other computer systems. The data crystal chip did not make an appearance in Halo 2. Instead, Cortana could transfer from the more advanced MJOLNIR Mark VI armor straight to any electronic system in direct contact with the suit. However, despite the armor in Halo 2 being the same suit as in Halo 3, the Data Crystal Chip makes a reappearance, and the direct-transfer system no longer seems to exist. It is possible that this was a deliberate choice to more resemble the transfer system seen in Halo: Combat Evolved, although it is also possible that it was merely for dramatic effect; in the opening cutscene, Johnson, thinking the Chief dead, is able to yank the chip out of the suit, and then ask the Chief about her whereabouts when he realizes the chip is empty.
 * A crystal chip is seen on some of the cafeteria tables on the Halo: Combat Evolved level Pillar of Autumn.
 * In Halo 3, the color of the crystal has changed to a blueish-purple hue as opposed to the aqua color of the Halo: Combat Evolved crystal.

List of appearances

 * Halo: Combat Evolved
 * Halo 3
 * Halo: Reach
 * Halo: The Fall of Reach
 * Halo: The Flood
 * Halo: First Strike
 * Halo: Fall of Reach
 * Invasion
 * Halo 4