As a crystal, quartz is used as a semiprecious gem stone. Crystalline varieties include amethyst, citrine, rose quartz, smoky quartz, etc. Cryptocrystalline forms include agate, jasper, onyx, etc. Because of its piezoelectric properties quartz is used for pressure gauges, oscillators, resonators and wave stabilizes; because of its ability to rotate the plane of polarization of light and its transparency in ultraviolet rays, it is used in heat-ray lamps, prism and spectrographic lenses. Also used in manufacturing glass, paints, abrasives, refractory materials and precision instruments.