The Orb of Aldur or Cthrag Yaska was at first, a ordinary looking stone found in a creek by the god Aldur. As a consequence of handling the stone through several centuries, the surface became smooth, spherical and highly polished, even becoming like a gemstone. The Orb was said to be about the size of a child's heart. Later when the will of the Orb awakened it glowed blue (possibly to reflect the color of the god Aldur.)
The Orb's main purpose was to be an intrument of the Child of Light and the Prophecy. It was embodied with the power of one of the two purposes of the universe. The other being the Sardion and by extension the Child of Dark. The Entity of one side of the divided purpose came to Aldur and told him of the arrival of the two stones. The sheer power of the orb alone is incalculable, but when combined with the will of a God or Child of Light the Orb will magnify the power of the user. The Orb was consided a living entity with a childlike will, and was characterised as having a naïve and slightly mischievous personality (such as spying of Belgarion and Ce'Nedra on their wedding night) and seemed to enjoy irritating its warden, Belgarion.
In his autobiography, Belgarath dismissed the story in the Book of Alorn that Riva Iron-grip was chosen to guard the Orb because he alone of his brothers and father was "pure of heart". In reality Riva was the one who took the Orb because the Orb had bonded itself to him, and Riva was the one destined to devote his life to guarding it.
The Angarak name for the Orb of Aldur was "Cthrag Yaska", which translates as "The Burning Stone", after its effects on the Angarak god Torak when he split the world using its power. As a consequence the orb would not permit itself to be used for harm again, and destroyed anyone who touched it with the exeption of Riva and his decendents, The Orb was intensely dedicated to the Rivan Line, and when Geran, the heir to the Rivan throne, was kidnapped, the Orb was able to act as an extremly accurate divining rod, using the blade of the Rivan King's sword to point and tug in the direction the heir was taken if it should pass over the path followed. The only drawback was, that the Orb could not trail Geran over water, as water is not fixed in place as land is.
The Belgariad-The Orb became the object of a recovery effort after its theft by Zedar the Apostate, from riva. He used the innocent child Errand who later became Eriond , to obtain it from Riva using him as a sort of living transportation device. Eriond is the only person besides Garion or Geran who is able to touch the Orb. Belgarath is able to detect the trail of the Orb.
The Orb was later recombined with the Sardion, and shards of the Sardion and the transformed husk of Zandramas' body were used to 'fix' the "hole in the universe" caused by the accidental explosion of a sun near a group of stars causing a tear or hole in the fabric of the universe, dividing the one purpose of the universe into two distinct entities. This event was known as the "Accident".
A new 'Orb' resulted in the combined Orb of Aldur and the Sardion, of which Garion is its guardian.