Flash Rocks

Certain quartz-containing rocks will emit a yellow flash when struck together. Generally speaking the best rocks to use are rounded white quartzite pebbles. When the rocks are struck together with a glancing blow, the tiny quartz crystals within the quartzite are compressed. These crystals apparently build up charges due to the piezoelectric effect. The charges recombine within the rock (sort of an internal sparking) producing yellowish flashes and sometimes a burnt sulfur smell.

ABOVE LEFT: Striking (in this case vigorously rubbing) two quartz-laden rocks together.

ABOVE RIGHT: A faint flash from these rocks. Photographing the flashes is extremely difficult; the flashes are best seen in person.