Sword of Truth
The Stone of Tears is a black stone that was hung around the Keeper's neck by the Creator in order to seal the Keeper in the underworld behind the veil. The Stone of Tears is one of the seals keeping the Keeper in the underworld, and will turn amber if touched by the tears of a wizard. It was the focal point of the book Stone of Tears.
If the Stone of Tears is in the world of the living, it means that the veil to the underworld is torn. The Stone of Tears is then required by someone with both the gifts of Additive and Subtractive Magic to repair the veil.
When Darken Rahl opened a Box of Orden, tearing the veil, the Stone of Tears appeared in the world of life. Zeddicus Zu'l Zorander gave Rachel the Stone of Tears on a necklace to wear and guard until Richard Rahl came for it.
Legend of the Seeker
In the TV series, the Stone of Tears had a rather different backstory and appearance. It appears as a small, pale blue stone which only changes color when it is hit by the light of the solstice sun. The Stone of Tears was made from the tears of the Creator when Her lover, the Keeper, tried to kill their children (humanity) ("Creator"). She used it to seal him away, then left the Stone in a far-flung corner of the mortal world, guarded by the eldest of Her children ("Eternity"). Within the series narrative, the Stone of Tears could only be found with a special compass, usable only by the Seeker. Prophecies kept by the Sisters of the Light warn, however, that the Seeker is destined to surrender the Stone to the "Enemy of the Light" (revealed to be the Keeper but also thought to be Darken Rahl or Sister Nicci). Ultimately, Richard is tricked into giving the Stone to the Keeper, prompting a Confessed Kahlan Amnell to kill him. The power of Kahlan's love for Richard and sorrow at her actions, creates a new Stone of Tears, which the Seeker is able to use to heal the rifts and seal the Keeper once more. ("Tears")