The Moonstone is a translucent milky-colored, flat ovoid gemstone that is approximately "the size of a hockey puck" (resembling milk quartz more so than true moonstone). The stone plays a key role in the overall plot arc of Season Two, and is sought after by several of the seasons antagonists. Later it was known that the stone is the magical focus of the curse placed on both vampires and werewolves. But ultimately it was known that this was only a hoax created by the Original Vampire Klaus. The actual curse was placed in him that sealed away his werewolf side as he is a Hybrid.
The stone is first mentioned, seemingly in passing, by Mason Lockwood, who tells his nephew Tyler that it is a Lockwood heirloom with only sentimental value. Tyler, obviously suspicious, locates the stone in one of his father's hiding spots and takes it. He initially intends to keep it away from Mason, but after learning of his werewolf curse, and nearly killing a girl by accident, he turns it over to Mason just to be rid of it. Mason reports his success to Katherine, who had apparently sent Mason after the stone.
Jeremy, who has seen the moonstone in Tyler's possession, informs Damon about it, and offers to retrieve it for him. Alaric locates some of Isobel's research, which reveals the importance of the moonstone—that it was used to seal the werewolf curse, and could be used to remove it. When Tyler explains to Jeremy that he has given the moonstone over to his uncle, Damon and Bonnie kidnap Mason and extract the stone's location from his mind. Bonnie then goes off to find Stefan and Elena, who manage to acquire the moonstone before Katherine.
Katherine responds by compelling Elena's aunt Jenna to stab herself with a knife, and threatens to kill everyone else in Mystic Falls until she gets the stone for herself. Stefan (correctly) guesses that Katherine originally stole the moonstone from someone else, and that it's part of the reason she had faked her death in 1864. Katherine gains possession of the moonstone, but is trapped in the tomb with it, until Jeremy manages to take the stone from her and throw it to Bonnie outside of the tomb, beyond Katherine's reach. Bonnie then takes the stone to a powerful witch friend, Luka, for help destroying it. Luka, however, fakes the stone's destruction, and instead secretly turns it over to his father, who has made a deal with one of the original vampires, Elijah, to acquire the stone.
In Crying Wolf, the stone is briefly seen when Elijah shows it to a group of werewolf torturing Damon for information of the stone. When one of then tries to take it from him Elijah kills him and then kills every werewolf in the room but one who escapes. This seems to show that Elijah now usually carries the stone with him all the time.
In The Dinner Party, after killing Elijah, Damon finds the stone in his clothes. Its was in the Salvatore's possession until Know Thy Enemy where Katherine stole to give it to Klaus thinking she could get pardoned by delivering it but it was just a elaborated plan to take her and Moonstone. Its currently in Klaus possession again.
In The Sun Also Rises, Klaus have use the moonstone for the ritual.Greta casts a spell that unbinds the "Klaus's Curse" which is sealed in the moonstone so as a result it's destroyed.
The moonstone was originally thought to be used some time in the 15th century, by an Aztec shaman, as the focus of a curse laid on both vampires and werewolves. The effects of the curse made vampires vulnerable to the sun, and tied the werewolves' transformation to the full moon. As a key part of their curse, the stone is considered immensely valuable to both vampires and werewolves who know of its history. With the correct combination of blood (from a werewolf, a vampire, and a doppelgänger of the Petrova bloodline), and the stone, a witch could reverse the curse. It is believed that moonstone's owner could control how the curse was removed; for example, a werewolf could leave the vampires' curse of the sun in place while removing the werewolves' curse of the moon.
It was later revealed that the Sun and Moon Curse was actually made up by Klaus. The moonstone was used to bind a curse that was placed on him by his mother, who was a witch, to suppress his werewolf side and to keep him from being truly immortal. He made up the Sun and Moon curse to get werewolves and vampires all over the world to look for the two keys to breaking the curse: the moonstone, and a human Petrova doppelgängers blood. The first Petrova doppelgänger, Katerina (Katherine) Petrova (Pierce), was discovered by him in 1492, and he charmed her into staying with him in order to sacrifice her, finally taking her captive when she learned the truth. Katerina escaped captivity, taking the moonstone with her, then tricked Rose into turning her, making her blood useless in the ritual. This resulted in Klaus killing Katherine's entire family in Bulgaria, and she has been on the run from him ever since.
In 1864, Katerina was now Katherine Pierce, living as a vampire in Mystic Falls, during the Civil War. During that time, many people were being murdered and drained of blood, and there was mass hysteria about vampires running rampant in the town. It was later revealed that it wasn't vampires killing the people, but werewolves. Katherine somehow knew this while either did not, and knew that the particular werewolf was George Lockwood, one of the Founding Families of Mystic Falls. Katherine made a bargain with him; she will give him the moonstone and keep his secret, and in return, he would expose the other vampires in Mystic Falls and make it appear that she was killed in the same manner. After the Founder's Council carried out its plan of massacring of all of the vampires in the town at George's behest, both Katherine and George mutually honored their bargain. George passed the moonstone down through his descendants, until it ultimately was in the possession of Richard Lockwood at the start of the series. It was recently part of a power struggle between vampires and the werewolves and then with the at least temporary defeat of the werewolves, between Klaus and Elena's protectors Stefan and Damon Salvatore and Bonnie Bennett, a witch who is a friend of the Salvatore brothers. As of Know Thy Enemy Klaus has possession of it. Finally in the episode The Sun Also Rises, Klaus used the stone to break the curse, and the stone is now believed to have been destroyed in the process.
- In the episode Plan B, Alaric mentioned that the portion of the werewolves on the curse is sealed in Moonstone but in the episode By the Light of the Moon, Bonnie said that the Moonstone has the curse of the sun and moon.
- It is unknown what happened to the Moonstone after the sacrifice but it can be assumed that it was destroyed.
- When Damon finally obtained the moonstone, he hid it in a bowl of decorative soaps in his bathroom to hide it from Katherine, but the plan is later proven unsuccessful when Katherine later found it while he was out of the house. However, it was quite clever since it easily blended in.
- The moonstone is a very pale white color in the show, but in real life the moonstone is more similar to the color of milky quartz.