Shell Necklaces are magical necklaces with a golden shell, given to the twins by the Seafoam Spirit to protect them from the Thornbush Pit. They are twins, and if one of them is damaged the other loses its magic too.
In The Wishing Spell, Alex and Conner need to go to the Thornbush Pit to retrieve the Little Mermaid's dagger for the Wishing Spell. The Seafoam Spirit gives them the magic necklaces so the vines will not attack them.
"She extended both of her hands, and two necklaces, each with a golden shell, appeared in them."