A holocron, short for holographic chronicle, was an organic crystal-lattice device, in which the Jedi, as well as the Sith, of old stored phenomenal quantities of data, as well as the holocron's gatekeeper. The Sith also had their own form of holocron technology, and they seem to predate the Jedi in using the technology by at least three thousand years.
Most holocrons were shaped like polyhedra; Jedi holocrons were typically cubical, while Sith holocrons usually took the form of a pyramid, but other geometric shapes also existed. The Telos Holocron represents an atypical shape. Normally a holocron could be held in one's hand.
The existence of the first Sith holocron predates that of a Jedi holocron by at least three thousand years. The technology to create the pyramidal holocrons common to the Sith was something gifted to the ancient Sith king Adas by Rakata warriors trying to gain his trust. He accepted their technology and made a holocron, but he saw through their ploy and rebelled against them.
The holocron employed patterns of organic crystals and hologrammic technology to capture the appearance and cognitive networks of the holocron's owner, and transform this into a three-dimensional hologram. This gatekeeper acted as a guide through the holocron's store of information, as a search and recovery engine with a more personal touch. These gatekeepers could relate stories from the holocron's archives to events in the life of the one who activated the holocron. Jedi gatekeepers could sense the ability and level of a Jedi, and offer or restrict information accordingly—so that Padawans were not shown information too advanced and dangerous for their basic stage. Sith holocrons usually had the same restrictive mechanism like Jedi holocrons, but the gatekeeper could corrupt the unawere listener.
A great many holocrons were stored in the Jedi Archives on Coruscant, kept in a specific Holocron Chamber within the Jedi Temple. In addition to holding the holocrons created by Jedi in the field, the Archives also held the details on creating holocrons and directions on accessing them.
The technology to make them was nearly lost after the Great Jedi Purge when the Archives were raided by Darth Sidious. Surviving holocrons contained information including Jedi techniques, blueprints of ancient starships, and historical information about the universe; as a result, they were quite valuable. Usually, only Force-sensitive users could access a holocron, while data holocrons did not require Force sensitivity to access. This suggests that holocrons were partially powered by the Force. Once opened, Force-insensitive users could communicate with the gatekeeper as well as Force-sensitive users. It is known that Bastila Shan's father made a holocron, which he used as a journal.
Some holocrons were activated by taking them apart with the Force and then inserting a special crystal that made the holocron readable. The crystal was usually hidden away for safe-keeping by the founder of the holocron, but there have been instances in which a certain Jedi or Sith has protected the crystal at all times by keeping it on their person.
Although not seen in the film, during production of Attack of the Clones, the film's propmakers made three holocrons for its Visual Dictionary: a Jedi holocron, a Sith holocron and the Great Holocron.
Housing holocrons at Jedi or Sith Temples is a way of generating nova crystals in Star Wars: Galactic Battlegrounds. They appear identical to the Vor'Na'Tu artifact in the game. They are equivalent to the relics of Age of Empires II, on which SWGB was based.
In Star Wars Galaxies, holocrons were used at first to unlock Force-sensitivity in users and later, when Aurilia was used for players to become Jedi, the holocrons were used to refill their Force bar. Two holocron collections were added to the game with Chapter 7 in November 2007.
In Star Wars: Jedi Knight: Jedi Outcast, Kyle Katarn trains on Yavin 4. There he collects holocrons that 'teach' him how to increase speed, Force jump and push or pull certain objects with the Force. In multiplayer mode, a player that collects a holocron can learn how to use a Force power along with new lightsaber abilities.
In the Force Unleashed video game, there are two kinds of holocrons; Sith Holocrons function as traditional "power-ups", granting Starkiller temporary invincibility, enhanced damage, limitless Force energy, etc.. Jedi Holocrons serve as hidden bonuses, commonly containing a supply of Force points (the game's equivalent of experience points), but also containing various lightsaber crystals or alternate costumes. In the Playstation 2/Nintendo Wii version of the game, holocrons unlock artwork from the levels in the game.
There are many different types of holocrons in SWG.
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These collection pieces are very rare and can be worth alot. The most common holocrons are the 3/5 pieces and are the least valuable of all. The rarest is the Strange Sith Holocron 4/5 (Waist Pack).
These holocrons grant you the Meditate ability and the Belt of Master Bodo Baas, a wearable item. You can complete these collections to activate another collection, the Master Jedi Cloaks, for the Cloak of Hate and the Shatterpoint Cloak.
Pre-NGE Holocrons were hints at the class unlocks you needed to complete in order to unlock your Jedi slot. Players accessing holocrons would be told - quite directly - a class. In order to 'complete' that holocron, you needed to master that class. Once done, accessing another holocron would inform you of another class. These holocrons are nowadays used for nothing more than decoration.
This used to provide access to the online knowledge base system to contact cutomer support amongst other things. Now it has changed to the Help system which is an online non in-game system.