embedded spark plug fragments, the ceramic, at least that's what I overheard while minding my own law abiding business having breakfast at Shoney's.
The real difficult part, or so I overheard, is the spring needed to generate the force needed to set off the primer, I did not hear of the other obviously dastardly people who were not related to me in any way by blood or association, apart from sharing the same species you see, had come up with a metal detector evading solution.