The Crystal went by several names, depending on the era. Prior to the arrival of the Fallen urSkeks, it was known as The Great Crystal. During the Age of Harmony, it was called The Pure Crystal and was renamed The Dark Crystal after The Great Division. Once the Crystal was healed, and the Division reversed, it was re-christened The Crystal of Truth.
The Crystal was quartz like in shape and was rombohedral at its top and base, with a threefold symmetry. It was originally encased within a mountain in the Bah-Lem Valley, which was later demolished by the Fallen urSkeks and reformed around the Crystal as a magnificent crystal fortress. The Crystal was suspended in mid-air by its own gravity above a shaft of air and fire within the Castle's triangular ceremonial chamber. The Crystal's molecular structure had a spiral linkage, allowing it to rotate the plane of polarization of a beam of polarized light.
During a Great Conjunction, the polarized power emanating from The Three Suns onto the Crystal would untwist the linkage and produce a focused and intensely concentrated beam. However, after the loss of the shard, the linkage could no longer be untwisted. The Skeksis discovered that the broken Crystal's damaged linkage could entrap malevolent, yet reinvigorating energies, which could prolong life. These energies affected the Crystal's molecular structure to the point of giving it a wine dark color.
The Crystal originated during the formation of Thra, and acted as the planet's "beating heart". Its energy reverberated throughout Thra, and it maintained a link to all living things inhabiting it. Some time later, sensing the sorrow of the rocks and trees over not having the ability to speak or see, the Crystal created Aughra to act as their voice and eyes.
Arrival and stewardship of the Fallen urSkeks
Nine hundred and ninety-nine and one trine after the formation of Thra, the Crystal was hit by the rays of the first Great Conjunction, which allowed the Fallen urSkeks to enter Thra. Promising "peerless gifts" to Thra's inhabitants, the urSkeks demolished the mountain encasing the Crystal, and formed a vast crystal fortress modeled on their home world around it. Although the majority of Thra's inhabitants appreciated the cultural and technological innovations brought by the urSkeks, they were resented by Aughra's son Raunip, who saw the urSkeks' taking residence in the Castle as an act of appropriation.
Inciting a mob of Gelflings to attack the Castle, Raunip confronted the urSkeks, demanding that they relinquish the Crystal. The urSkeks replied that it belonged to all of Thra, and was not theirs to give. Further escalation was prevented by Aughra's intervention.
The Great Division
The Fallen urSkeks, with Aughra's assistance, planned to use the light of the Crystal to purify themselves of their darker natures and return home during the second Great Conjunction. The urSkeks created a network of mirrors around the Crystal, which would focus the rays of the Three Suns shining down on the Crystal onto each of them, thus burning away the evil in their souls. However, when the Great Conjunction began, one urSkek gave into its darker nature, resulting in The Great Division of the urSkeks into Skeksis and urRu.
In the ensuing confusion, a Skeksis attacked the Crystal and chipped off a shard. The resulting sound was said to have rendered the Skeksis tone deaf, and was thought to still be audible within the Castle centuries later. The impact also caused an earthquake, which spread beyond the Bah-Lem Valley and struck the Podling village of Noy, killing clan mother Hakmeena.
Stewardship of the Skeksis
The Skeksis took control of the Castle, proclaiming themselves "lords" of the Crystal. They initially tried to create a Crystal of their own, but only succeeded in building useless black rocks, which they reused for their Trial by Stone tournaments. Although initially benevolent in their rule, the Skeksis became increasingly violent as they became aware of their own mortality.
They initiated The Ceremony of the Sun, which made use of sun rays from lesser Conjunctions hitting the Crystal in order to prolong their lives, and further used the Crystal's beams shining down the shaft of the Castle to drain the essence of Podling and Gelfling slaves.
The Third Great Conjunction
The Dark Crystal is healed by Jen during the Third Great Conjunction, the Mystics and Skeksis are made one, becoming the urSkeks. The Crystal once again becomes the Crystal of Truth.