The shapeshifting locket was an accessory the Rakhari Croden was wearing when he visited Deep Space 9 in 2369.
He told the security chief Odo, a shapeshifter, that he got this locket from a colony of shifters who once lived on his homeworld of Rakhar. In fact this locket was a key to a stasis pod in which Croden's daughter Yareth was kept until Croden released her. (DS9: "Vortex")
The shapeshifting locket was designed by Rick Sternbach who drew the sketch for this prop. (DS9 Season 1 DVD special feature "Deep Space Nine Sketchbook")