Box Clasp
A folded metal tongue that clicks into a box. The traditional fine-jewellery clasp, usually with a safety catch.
A sprung tongue is pushed into a box housing and locks when it expands inside. Good ones add a figure-eight safety catch as a second line of defence.
Because the box can be decorated and set with stones, it is often designed to be worn at the side or front as part of the piece.
The tongue can fatigue over decades of flexing and eventually needs replacing, which any competent jeweller can do.
When you are buying
Always use the safety catch. It exists because the tongue is the part that fails, and it is what stops a strand hitting the floor when it does.
Clasp
The fastening that closes a necklace or bracelet. The most common point of failure and the most overlooked part of a purchase.
Lobster Clasp
A spring-loaded hook shaped like a lobster claw. The most secure everyday clasp and the modern default.
Screw Clasp
Two halves that screw together, usually shaped as a small ball. Traditional on Indian pearl necklaces and very secure.