Shocked quartz
Quartz crossed by fine parallel lines, formed only by extreme, sudden pressure such as a meteorite impact. It is a fingerprint of impact craters.
Shocked quartz is ordinary quartz that has been hit by a shock wave so strong that its crystal structure is damaged along sets of fine, parallel planes. Under a microscope these show up as thin straight lines across the grain, called planar deformation features.
It forms at pressures of more than about 2 gigapascals, roughly twenty thousand times the pressure of the air around us, delivered in a fraction of a second. Meteorite impacts and nuclear explosions can do this. Volcanic eruptions, however violent, do not generate the required pressure.
That is why shocked quartz was decisive in two famous cases: in the thin clay layer that marks the end of the age of dinosaurs, and in drill samples from the buried Chicxulub crater in Mexico.
A common misconception is that heat alone makes it. It is pressure, applied suddenly, that matters: slow pressure deep inside the Earth bends and recrystallises quartz instead.
