Resurrection stone

The Resurrection Stone was one of the Deathly Hallows and one of Voldemort's Horcruxes. According to legend, whoever unites it with the Elder Wand and the Cloak of Invisibility will be the Master of Death. Created by, or given by Death to, Cadmus Peverell, the second of the Peverell brothers, it had the power to bring people back from the dead, but not to render them as if they had never died - that is, as clearly defined spirits, not as living people. On its surface is drawn the symbol of the Deathly Hallows: a circle inscribed in a triangle, down the middle of which ran a line.

History
"Then the second brother, who was an arrogant man, decided that he wanted to humiliate Death still further, and asked for the power to recall others from Death"

- "The Tale of the Three Brothers"

Legend has it that Cadmus Peverell, the first owner of the Stone, used the Stone in a desperate attempt to bring his beloved back from the dead; he succeeded, but only in that he managed to bring back a ghost-like form of her that did not truly belong in the world. Driven mad from the knowledge that he could never bring her back fully, he killed himself. The Resurrection Stone was kept in the Peverell family, passed down from generation to generation, eventually finding its way to the hands of Marvolo Gaunt, a descendant of the Peverells, in the form of a ring. Apparently, neither Marvolo nor any of his relatives knew the true nature of the ring or the symbol scratched upon it; Marvolo once claimed that the sign was the coat of arms of the Peverells. In 1925, Marvolo and his son Morfin were arrested and sent to Azkaban for attacking Muggles and resisting arrest; the strain of imprisonment and the subsequent flight of his daughter Merope proved too much for Marvolo, and he died soon after he was released, bequeathing the ring to Morfin. In 1943, Tom Marvolo Riddle, soon-to-be Dark wizard Voldemort and illegitimate son of Merope, stole the ring from Morfin while in Little Hangleton murdered his father and grandparents. Morfin never realized that the stone had been stolen from him, his memory having been expertly modified by Riddle, but he regretted the loss of the ring to the end of his days.

Sometime during Riddle's quest for power and immortality, the ring and stone were turned into a Horcrux, an object made to contain a fragment of a wizard's soul, by killing his father and paternal grandparents. As was conjectured by Albus Dumbledore, Riddle may not have known the true power and nature of the Stone, otherwise he would not have turned it into a Horcrux so readily. Whatever the case, Riddle replaced the ring in the Gaunt shack and protected it with powerful enchantments, assured that it would be kept hidden forever. This was not the case, as in the summer of 1996, Dumbledore managed to break through the spells, take the ring, and destroy the part of it that contained Riddle's soul; the power of the Stone remained, however, intact. Before he died, Dumbledore hid the Hallow in a Snitch and requested in his Will that it be given to Harry Potter. When Harry placed the Snitch near his mouth, a message appeared that said "I open at the close." At first, Harry was unable to work out what this clue meant, and he carried the Snitch inside his mokeskin pouch during the months he searched for Voldemort's Horcruxes.

Harry used the Stone only once. After realizing that it was necessary to allow Voldemort to kill him, he finally understood what the words on the Snitch meant. Placing the Snitch near his mouth, he whispered "I am about to die," and the Snitch opened, revealing the Resurrection Stone within. Harry turned the stone three times in the palm of his hand, bringing back his parents, Sirius Black, and Remus Lupin for emotional support before he sacrificed himself to Voldemort. Upon reaching his nemesis' camp in the Forbidden Forest, he dropped the Stone into the leaves and lost it; he later said that he had no desire to return and find the Stone. The Stone presumably lies in the Forest to this day, never to be found again. Rowling later revealed that it was pressed into the ground by a centaur's hoof.

Properties and usage
The Stone was held to be mythical by many wizards, who believed it to be no more than a fairy tale created by the storyteller Beedle the Bard. Indeed, the original mention of the Stone is in one of Beedle's tales, The Tale of the Three Brothers, which relates the adventures of three brothers who outwit Death, receive gifts (the Hallows) from him, and eventually lose their lives to him one way or another. Learned wizards believe that the Deathly Hallows were in fact created by the three highly skilled Peverell brothers, and that Beedle's tale is just fact passed on as legend.

To establish a connection with the dead, the Stone must be turned thrice in one's hand, at which the summoned spirits will appear. These spirits are not memories, such as those created by Priori Incantatem, or ghosts, spirits of wizards who have not made the journey to the "beyond"; neither are they living people or solid bodies, as no spell or deep magic can totally bring back the dead.

The Stone was used at least twice; when Cadmus Peverell brought back his loved one and when Harry Potter called back Lily Potter, James Potter, Sirius Black, and Remus Lupin.

Behind the scenes

 * Despite posessing it for most of their lives, it is unknown whether the Gaunt family ever used the stone or not. It is never made clear whether the stone must be removed from the ring to use it or if turning the ring over in your hand three times would suffice. If it was used, it could explain much of the dementia and family pride Marvolo experienced, having been spurred on by his dead ancestors.
 * This is highly unlikely however, given that Marvolo did not even recognize the symbol on the stone for what it really was, indicating he had no clear idea what the symbol truly meant and by extension likely did not know the stone's true power. In addition, the mental instability exhibited by Marvolo and Morfin was said by Albus Dumbledore to have been caused by generations of inbreeding by the family.