When struck together in the dark a glow appears from inside the rock.
The reason for the glow
Scratching the rocks together separates the positive and negative charges within the quartz. Enough volatage is obtained to cause an electric arc. Then the two charges re-join to neutralize each other.
This phenomenon is called the piezo - electric effect and it has several important technological uses.
source stone science. August 08.
piezo - electric
The piezoelectric effect was discovered in 1880 by the Jacques and Pierre Curie brothers. They found out that when a mechanical stress was applied on crystals such as tourmaline, tourmaline, topaz, quartz, Rochelle salt and cane sugar, electrical charges appeared, and this voltage was proportional to the stress.
First applications were piezoelectric ultrasonic transducers and soon swinging quartz for standards of frequency (quartz clocks).
An everyday life application example is your car’s airbag sensor. The material detects the intensity of the shock and sends an electricla signal which triggers the airbag.
source :http://www.piezomaterials.com/ september 08
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