Ninja rocks are broken shards of spark plugs 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.
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How they work
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. 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.
California
In California, since 2003, ninja rocks are explicitly listed as burglary tools, and their possession with intent to burglarize is a misdemeanor punishable by up to six months in county jail and/or a fine of up to $1000. Legal records do not use the phrase "ninja rocks", preferring more precise phrases such as "ceramic or porcelain spark plug chips or pieces".
Until 2003, "burglary tools" in California did not include devices to break glass. In late 2001, two important convictions including possession of ninja rocks were appealed. In People v. Gordon (2001) 90 Cal.App.4th 1409 (Review denied), Division 1 (San Diego) of the Fourth District Court of Appeal found that possession of ninja rocks was not punishable under section 466 of the penal code. That court applied the ejusdem generis rule of construction, deciding that ninja rocks were not enough alike the then-listed burglary tools. On the other hand, in In re Robert B. (2001) 93 Cal.App.4th 963, Division 3 (Orange County) contradicted this interpretation of section 466 and upheld the conviction. On February 13, 2002, the latter case was granted review by the California Supreme Court.
Two days later, the state assembly proposed in Assembly Bill 2015 to amend section 466 to include ninja rocks. The bill passed unanimously in both houses in August 2002.
Washington (state)
One Washington trial court found that the ability of ninja rocks to quietly break tempered glass meant that their possession could be used to establish intent to commit burglary, even in a case where the ninja rocks were not actually thrown at any glass because the burglars had found an unlocked door. One defendant appealed his conviction to the Court of Appeals on the grounds that "the trial court erred by admitting an unusual burglary tool into evidence". The Court of Appeals denied this reasoning and upheld the conviction.