For 10 points, name this crystalline form of silica, a mineral with chemical formula SiO2 and a Mohs hardness of seven.
Varieties of this mineral include onyx, jasper and chalcedony, and it is one of the few minerals to feature conchoidal fracture. Its piezoelectric properties have resulted in its being used in clocks and watches. After feldspar, it is the most (*) common mineral on Earth, and it is the most common on the Earth's surface. Common varieties of it include citrine and amethyst. For 10 points, name this crystalline form of silica, a mineral with chemical formula SiO2 and a Mohs hardness of seven.
Varieties of this mineral include onyx, jasper and chalcedony, and it is one of the few minerals to feature conchoidal fracture.
{quartz}