From the Story
"This opulent necklace carries a powerful curse that has killed nineteen Muggles."
The necklace first appears when Harry accidentally ends up in Borgin and Burkes after his first Floo Network experience in The Chamber of Secrets.
In the Half-Blood Prince it returns as one of the attempted assassination devices Malfoy tried to give to Dumbledore via proxy. However, due to a hole in Katie Bell's glove, her hand just barely touched it, causing her to fly in the air. When Katie dropped the necklace, Harry immediately recognized it and stopped his friends from touching it directly, and later took it up to the school.