Screw Clasp
Two halves that screw together, usually shaped as a small ball. Traditional on Indian pearl necklaces and very secure.
A screw clasp cannot open accidentally, because it takes several deliberate turns to release.
It is often made as a decorative bead so the clasp disappears into the strand and the necklace can be worn with the clasp at the front.
Its drawback is fiddliness. Fastening one behind your own neck takes practice.
When you are buying
Best choice for a mangalsutra or any piece worn daily and rarely removed. Poor choice for something you take on and off several times a day.
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.
Mangalsutra
The necklace a married Hindu woman wears, traditionally black beads on gold with a distinctive pendant.