Quartz constitutes more than 12% of the earth's crust and is one of the most occurring minerals on the planet. It is used in the rotating mechanism of wrist and wall watches because it is hard enough to withstand constant wear. It is an important component of medical and scientific equipment, such as oscillators, frequency stabilizers and ultrasound generators. The mineral is also used as an abrasive in detergents and toothpastes. Although colorless when pure, quartz can be almost any color due to the impurities that settle inside the crystal. This makes it a popular gem. Amethyst, violet in color, is a variety of quartz. Quartz occurs in white, brown, yellow, black, pink or blue colors and is composed of oxygen and silicon. Although silicon is a black metal, it combines with oxygen to form an absolutely colorless mineral. In 1880, French physicist Pierre Curie discovered that quartz is piezoelectric: when put under pressure from different directions, it produces an electrical discharge on the surface. And when sawed or hammered, it emits a yellow glow that makes it visible in the dark.