It is an essential component of igneous and metamorphic rocks. The size varies from specimens weighing a metric ton to minute particles that sparkle in rock surfaces. In pure form, quartz is colorless, but it is commonly colored by impurities.
Rock crystal is a colorless form of quartz occurring in distinct crystals.
Rose quartz is coarsely crystalline and colored rose red or pink. Smoky quartz occurs in crystals ranging from smoky yellow to dark brown. Amethyst, a semiprecious variety of quartz, is purple or violet. Quartz also rotates the plane of polarized light and is used in polarizing microscopes. Different forms are also used as gemstones.