The Opal necklace is a cursed Dark object that was, for a period of time, on sale at Borgin and Burkes. It was claimed to have taken the lives of nineteen different Muggles. It was also used by Draco Malfoy in a failed attempt to assassinate Albus Dumbledore, that ended up cursing Katie Bell instead.
The necklace has a very deadly curse placed on it. When it makes physical contact on the victim, the curse would spread rapidly on the recipient, the level of the effect dependent on the amount of contact made; if simply brushed against a small amount of skin, then the victim would enter a trance of emptiness, levitate gracefully but eerily six feet above, then suffer such anguish that they will scream and thrash so wildly that they cannot recognise anything around them and fall back down, and if left untreated by a powerful enough counter-curse, it would lead to the curse spreading further and eventually death. Even after such treatment has been administered, it would take the victim around six months to fully recover at an institute such as St. Mungo's Hospital for Magical Maladies and Injuries.
Should a person be unfortunate enough to hold the necklace barehanded or wear it, it would cause death, perhaps instantly. Preventing the curse from affecting the holder would require avoiding any direct contact with the necklace, such as gloves or packaging paper.
When Harry Potter mistakenly went to Borgin and Burkes instead of Diagon Alley, he saw the necklace and read the label, "Do not Touch! Cursed. Has claimed the lives of nineteen Muggle owners to date."
- "At once, Katie rose into the air, not as Ron had done, suspended comically by the ankle, but gracefully, her arms outstretched, as though she was about to fly. Yet there was something wrong, something eerie... Her hair whipped her by the fierce wind, but her eyes were closed and her face was quite empty of expression... Then, six feet above the ground, Katie let out a terrible scream. Her eyes flew open but whatever she could see, or whatever she was feeling, was clearly causing her terrible anguish. She screamed and screamed..."
- —Effects of the opal necklace on Katie Bell[src]
In 1996, four years later, the necklace was purchased at Borgin and Burkes by Draco Malfoy, under the orders of Lord Voldemort.  During the first Hogsmeade trip of the 1996–1997 school year, Draco Malfoy cast an Imperius Curse on Madam Rosmerta. She in turn cast another Imperius Curse on student Katie Bell. Bell was ordered to take a package containing the opal necklace to Albus Dumbledore. Dumbledore would later comment to Malfoy that he believed this to be a crude and desperate method of assassination.
Harry Potter, Ronald Weasley, and Hermione Granger witnessed Bell arguing with her friend, Leanne, as they were heading back to Hogwarts. They noted that Katie did not seem 'herself,' acting dazed and detached. During the course of the argument, the package ripped, and the necklace touched Katie's skin through a minuscule hole in her glove. Katie rose gracefully into the air, screaming, with Leanne screaming as well and attempting to pull her down. Ron and Hermione attempted to help while Harry ran off to find someone, bumping into Hagrid and leading him back to the group. After learning the nature of the event, Hagrid carried Katie to the castle, accompanied by Harry and his two friends, who handed over the necklace to Professor Severus Snape, through the Caretaker Argus Filch. Katie was taken to St Mungo's Hospital for Magical Maladies and Injuries, where she spent much of that year recovering from the effects of the curse.
- Harry Potter and the Chamber of Secrets (First appearance)
- Harry Potter and the Chamber of Secrets (film)
- Harry Potter and the Half-Blood Prince
- Harry Potter and the Half-Blood Prince (film)
- Harry Potter and the Half-Blood Prince (video game)