An Iyaaran holographic projection device disguised as a necklace pendant
A holographic projection device was a form of holographic technology used by the Iyaarans in the 2370s. In appeared as an insignia in its normal form, but also appeared as a pendant on Anna's necklace. The Iyaaran Ambassador Voval wore the device to disguise himself as Anna, a female Human who was encountered by Jean-Luc Picard aboard a crash landed Terellian cargo freighter on a class M planet in 2370.
When an upset Anna attempted to leave the freighter, upset that Picard did not love her, Picard accidentally tore the necklace off her neck, before he dropped it by the fire. After being discovered by Voval, he and Picard went out to search for Anna. Picard became suspicious when he noticed that she was once again wearing the repaired necklace that had been left behind. Initially Picard thought that Voval had returned it to her, before it was finally revealed that Anna was Voval, and that the necklace was what allowed him to assume her appearance. (TNG: "Liaisons")
Later, upon meeting Anna, it is noted: "She wears a distinctive-looking NECKLACE with a pendant around her neck."
Following the breakage of her necklace and the reappearance of Voval, there is a parenthetical, "NOTE: The small insignia pin seen earlier on the Pilot's sternum is now gone – this is a subtlety that neither we nor Picard should notice."
Finally, as she is about to reveal herself as Voval, "She steps away from the ledge… and touches the pendant on her necklace – ANNA TRANSFORMS INTO THE PILOT, VOVAL. The necklace is now gone, and in its place we see that Voval is touching the small insignia pin at his sternum, which is actually an alien holographic projection device."