Ninja rocks are broken shards of spark plugs. Since they can quickly and almost silently fracture the glass windows on most cars, ninja rocks are increasingly the tool of choice in “smash-and-grab” auto burglaries. They have no traditional association with the ninja or ninjutsu, only being named such due to their “silent but deadly” function in burglaries.
How they work
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”.
Hat tip to An American Weasel.
Previous WOTD – Dunning-Kruger Effect and Anosognosia