JESUS, STOP CUTTING YOUR THUMBNAIL SO SHORT.
This is the Guillotine Ring (aka The Divorce) from Vagabond Jewelry. You have to constantly hold its little chain in your hand because if you let go it'll cut your finger off. No, no it won't. You will bleed though. And bleeding, my friends, is how you find out what your own blood tastes like. *peeling scab* Mmmmm, iron and alcohol.
Thanks to Charlotte, who cuts her fingers off the old fashioned way: playing with the garbage disposal.