This is a chain choker with an O-ring. It comes in 3 different sizes (15", 15.5", 16"). There is not adjustable sizing so I would recommend measuring your neck beforehand. Instead, it comes with a toggle clasp in the back for easy closing and opening (and much more user friendly for those with nails).
Chain: 6mm thick stainless steel and hypoallergenic
O-ring: silver alloy
Comes packed in a black jewelry box as shown in last photo. Perfect for gift giving or something special for yourself.