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 recognize 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.