This necklace is a commissioned piece to substitute for one that went missing. The main links are 20 by 25 mm and the whole piece weighs 100g. It has a toggle bar clasp which allows some adjustment of the length to suit different clothes. The bar can go through any of link of the other end of the chain, though it does then get a little asymmetric if it is shortened too much.
This picture gives a closeup of the links and the assay markings.