This has probably been done to death...
Is there a pen on the market, of the click ball point type, which is actually a spike, a stabbing weapon? If you were searched, what's the chance this would pass, if the spike was not exposed?
And how about a ring, with a cap (looks like a class ring maybe), spring open the cap, it exposes a spike? Is that strictly James Bond? Of course it would be very short, like a thumb tack, but still effective...
Sam