“Ninja rocks” are a burglary tool that allow people to quickly and (relatively) quietly break into vehicles by shattering the glass on the side windows of a vehicle.
Ninja rocks are broken shards of spark plugs that are formed by smashing the ceramic portion of the spark plug with a hammer or other large object. Since they can quickly and quietly fracture the glass side windows on most cars, ninja rocks have been used in “smash-and-grab” auto burglaries since at least 1995. They have no traditional association with the ninja or ninjutsu, only being named such due to their “silent but deadly” function in burglaries.
Tempered glass, which is used for the side windows of most vehicles, is manufactured with an extremely high surface compressive stress and high internal tensile stress, giving it strength and durability, but also leading it to abruptly shatter into thousands of tiny pieces when it breaks. When thrown with moderate speed at a side-window, a sharp shard of the exceptionally hard aluminium oxide ceramic used in spark plugs focuses the impact energy into a small enough area without blunting to initiate cracking, releasing the internal energy and shattering the glass.[7] However, ninja rocks are ineffective if the shards are insufficiently sharp, thrown with too little energy, or thrown against windshields, as these are made of a laminated type of safety glass, and therefore do not shatter.
This YouTube channel has a nice 3 minute video showing how the “ninja rocks” are created from shattering a spark plug with a large hammer, and how easily the small shards can destroy a side window.
Nice to know there are criminals out there using the physics of tempered glass to ply their trade.