This article may be confusing or unclear to readers. (July 2020) (Learn how and when to remove this template message)
Ninja rocks is a colloquial term for an improvised weapon or tool consisting of the extremely sharp porcelain or ceramic shards recovered from smashing or crushing the alumina insulator of a commercial spark plug. When thrown, ninja rocks are known to exploit the tensile stress present in the side windows on most cars in order to instantly shatter them, providing a quick and quiet alternative to other window-smashing methods and making ninja rocks ideal for emergencies or "smash-and-grab" auto burglaries, having been used in the latter function 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 and a superficial resemblance to the shuriken stereotypically used as a throwing weapon by ninjas.
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. This is to avoid the dangerous shards of normal glass in a car accident. 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.
The examples and perspective in this section may not represent a worldwide view of the subject. (April 2010) (Learn how and when to remove this template message)
United States Of America
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.
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.
- Ashton, David F. (November 30, 2005). "Valuables in car? Thieves are "smashing and grabbing"". East County News.
- Bridgewater, Carroll C. (1999). "State of Washington v. Andrew Mcmanus: Opinion Information Sheet". FindLaw. Retrieved 2006-07-30. CS1 maint: discouraged parameter (link)
- Tenner, Edward (2003) . "The dark side of tinkering". Knowledge, Technology & Policy. 16 (2): 42–44. doi:10.1007/s12130-003-1025-5.
- McNamara, Danielle (November 28, 2005). "Officers fight auto theft with stepped-up patrols". Contra Costa Times.
- Sonoma Police Department (April 2006). "Police Blotter". Sonoma Valley Sun. Archived from the original on 2006-06-02. Retrieved 2006-07-30. CS1 maint: discouraged parameter (link)
- Tenner, Edward (2001-12-24). "The Dark Side Of Tinkering". U.S. News & World Report. Retrieved 2006-07-30. CS1 maint: discouraged parameter (link)
- National Geographic demonstration video
- "Documents associated with AB 2015 in the Session". California State Legislature. Retrieved 20 June 2016.
- Judicial Council of California (2002). "2002 Legislative Summary" (PDF). Retrieved December 20, 2005. CS1 maint: discouraged parameter (link)
- Amended version of section 466 on FindLaw
- San Diego Legal Updates: Burglary Tools