What episode of the Twilight Zone where a doll loses his necklace and becomes alive.

This one?

Living Doll (The Twilight Zone) - Wikipedia, the free encyclopedia

OR

This one?

(not Twilight Zone) Trilogy of Terror II is a 1996 made-for-cable sequel to the 1975 television film, Trilogy of Terror

Story III: He Who Kills

This segment, about the Zuni fetish doll "He Who Kills", is a sequel to the third segment of the original Trilogy of Terror, "Amelia".

After finding the double homicide of Amelia and her mother from the first movie with the doll at the scene, the local police drop off the doll to local Dr. Simpson (Lysette Anthony). As she begins to examine the doll she learns that the doll comes to life when a gold chain is removed from his neck and that the doll has a desire for flesh. It also seems to regenerate (the idea itself initially laughable to both Dr. Simpson and her assistant) as when she chips away the charred wood, the doll seems to be brand new.