This beautiful necklace is made from black Vauxhall glass which originated in the Vauxhall area of London in the middle of the 17th century. It is a mirrored glass, that’s is glass backed with foil which makes it very reflective. It is mounted on black lacquered base metal or brass. It is often thought to be the English version of French jet (both popular substitutes for natural Whitby jet) but can be told apart by the red flash it exhibits under direct light. 
This choker length necklace features a river of faceted round links with a hexagonal and teardrop pendant drop which can be removed via the hook on the back if so wished. It has a simple hook clasp and dates from the very end of the 19th century. 
In good antique condition. Some light wear to the glass as to be expected but nothing obvious that detracts from its beauty. Some deterioration to the surface of the back, not visible from the front.
Length around neck: 34.5cm / 13 5/8”
Length of drop from top of round link: 4.8cm
Round link diameter: 9.6cm
Weight: 26.5g