Ninja rocks are broken shards of spark plugs. Since they can quickly and almost silently[1][2] fracture the glass windows on most cars, ninja rocks are increasingly the tool of choice in "smash-and-grab" auto burglaries.[3][4][5] 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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The effectiveness of the ceramic shards is a result of the hardness of the material relative to tempered glass and the structure of the glass. Tempered glass used in automobile windows is softer on the Mohs scale than spark plug ceramic (specifically aluminum oxide ceramic). Aluminum oxide ceramic rates a 9 on the Mohs scale; diamonds are 10, glass is 6.5. Additionally, tempered glass is manufactured with an extremely high surface tension, giving it strength, but also causing it to abruptly shatter into thousands of tiny pieces when it breaks. The result is that the hard, sharp shard of ceramic creates a small scratch in the glass which then rapidly propagates across the entire piece of glass as the surface tension is released. The glass subsequently shatters into many small pieces.
In California as of 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 rendered the grant of review moot by proposing in Assembly Bill 2015 to amend section 466 to include ninja rocks. The bill passed unanimously in both houses in August. Brian Franks notes that AB 2015 was concurrent with a "legislative flurry after Sept. 11, 2001".[6]
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 burgle, 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.[2]