The Firestone, worn as a necklace. TV: The Unicorn and the Wasp
A necklace was a type of jewellery worn around the neck.
On Earth, Cessair wore the Great Seal of Diplos she had stolen, which was in actuality a segment of the Key to Time, as a necklace. (TV: The Stones of Blood)
The Kinda wore necklaces embossed with a design akin to the DNA double-helix. (TV: Kinda)
In 2001, Dave Young bought Anji Kapoor a gold necklace with her name hanging from it as her anniversary present. It was the last present he ever bought for her before his death. (PROSE: Escape Velocity) She was still wearing it when she arrived on New Jupiter; when she was captured by guards, they decided that her necklace was proof that she was a member of ANJI and took it. (PROSE: EarthWorld)
A memory hub was a form of Ice Warrior recording technology that resembled a necklace. (AUDIO: The Bride of Peladon)
After defeating the Zeronites on Hippocrates Base, Destrii obtained a necklace which broadcast a holographic image of a human around her body. With it, Destrii was able to appear human at will. (COMIC: Sins of the Fathers)
Sebastian Grayle, under the guise of "Leofric of Exeter" in 1055, gave King Edward and Queen Edith exquisitely crafted necklaces made of plutonium. He did this so the radiation would kill them and then attack their castle in the ensuing chaos. (AUDIO: Seasons of Fear)
The Firestone was a powerful telepathic recorder and transmitter. It was given by the Vespiform Christopher to his lover, Lady Clemency Eddison, who wore it on a necklace. (TV: The Unicorn and the Wasp)