Onyx and Amethyst are both varieties of QUARTZ, a mineral composed of silicon dioxide. Onyx is a black or dark-colored QUARTZ, while Amethyst is a purple-colored QUARTZ. Both are used in jewelry and other decorative items. Onyx is often used to make cameos and intaglios, while Amethyst is used to make beads and cabochons. QUARTZ is also used in electronics, such as watches and clocks, and in the production of glass.