This is a distinctive necklace with a pentagram as its centrepiece in a design often known as the Devil's trap because it is said to be able to trap devils in a way that they can't exit.
Allegedly, this mystical symbol can immobilse and control demons, banish devils and stop you getting possessed by the devil himself.
The Devil's trap is approximately 20mm in diameter (just under an inch). The adjustable metal chain is about 65cm long.
Whilst I hope that you won't trap any real devils (apart from anything else, the PC brigade would probably object), this design is sure to be noticed.