This is a little crude but it is a bolt face of an AR-15.
I think this jewelery idea is interesting, but the jewelry itself is rather crap IMHO. While looking for something else I did find this - which I thought looked more like a work of art. Same idea - but more refined.
source: http://www.foxbusiness.com/personal-finance/2011/11/18/jewelry-made-from-ak-47s-for-greater-good/
