Why are spark plugs so strong that they can even be used in CNC machines?
The white spark plug insulator is actually made from alumina ceramic, or aluminum oxide.
Alumina ceramic has high hardness and is resistant to heat. This hardness level refers to its ability to withstand scratches.
Due to these properties, many creative people repurpose used spark plugs into useful tools.
Despite being very hard, alumina ceramic is still brittle like diamond.
