Ursula's Necklace is an enchanted nautilus shell belonged to the sea witch Ursula who wore it as a necklace and it is an object is featured in Disney's The Little Mermaid.
It was used to capture Ariel's voice as part of the deal to grant her legs. The movie implies that it may be a magical item as it is often glows when Ursula seems to start using a spell such as taking Ariel's voice or even transforming into Vanessa. On a semi-related note, there were also ghostly, seemingly-skeletal hands that originated from the nautilus, as evidenced when Ursula took Ariel's voice as part of the deal.
Close to the end of the movie, at a sailing ship where this false wedding between Vanessa and Prince Eric was held and halted during a sunset, Scuttle snaps the necklace of Vanessa's neck, helped largely by Prince Eric's dog Max when he bit Vanessa on the rump. It falls on the ship's deck and shatters, thus giving Ariel back her voice and freeing Eric from Ursula's spell.