The Stone Disc is one of the most holy objects in the two worlds. Set with four 'Dragon's Eye' jewels (blue, green, red and white), the Disc was brought to Earth by the Draic Kin. When complete and with the jewels in place, the Disc acts as the key to the Guardian's Tower; a replacement for the original disc used in the creation of the Divide.
According to Abnaxus, the Stone Disc had recently been mentioned at the Ayrede High Council. Apparently, the Tyren ambassador wanted to know where it was kept. No one at the Council admitted to knowing where it was. The Tyren ambassador was told to speak to the Sentinel about it, but Abnaxus thought it was as unlikely he would condescend to do so as it was that the Sentinel would respond. As the Tyren often act as the martial arm of the Vanguard in Arcadia, it seems likely the Tyren were trying to track the Disc down on their masters' behalf.
The Disc was kept at the Enclave for thousands of years, but after an attempt was made to steal the Disc long ago - an attempt conducted by Tyren soldiers and Sentinel traitors, chronicled in The Scriptures of the Balance (VIII, 16), and foiled by the Mother breaking the rule against Kin intervening - it was divided for safekeeping. Each of the Kin took one of the jewels, and the Disc itself was separated into four pieces. Four of the Magical Races of Arcadia - the Banda, the common Maerum and Alatien ancestor, the Dark People and the Venar - were entrusted with the pieces to await the coming of one who would reconstruct the Disc. This turned out to be April Ryan, though in some cases she had to go through several trials to prove herself.
The Disc had to be reassembled before the jewels could be added. In order to reconstitute the Disc, April returned to the Enclave and placed each piece at points around the edge of a diagram inscribed below the dome. After Crow reluctantly removed the pigeon guano clogging the mouth of the stone dragon overhead, it blasted blue fire - presumably magical - that melted the fragments into a single piece. It was then dropped into the pool in the centre of the Library beneath. Although she had already collected the blue and white in Arcadia, the red and green jewels were not added until April was in Jacob McAllen's laboratory.
When April arrived at the Tower, she placed the Disc into the Well of Making at its base, which caused the imprint of a hand to form out of the water. It did not react to April's touch, but responded to that of former Guardian Adrian. It is unlikely that the Disc will ever be retrieved, as its use has now been fulfilled, and the office of Guardian - along with the Tower - will not exist for much longer.