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. Because it is a primary ingredient in sand it is also used to make glass, silica brick, cement, and mortar. Ground quartz is used as an abrasive in stonecutting, sandblasting, and glass grinding.
Powdered quartz is used in making porcelain, scouring soaps, sandpaper, and wood fillers.