<p><strong>Noun:</strong></p><ul><li>One of the commonest minerals, silicon dioxide (SiO₂), having many varieties that differ in color, luster, etc.</li><li>Occurs in masses (e.g., agate, bloodstone, chalcedony, jasper) or in crystals (e.g., rock crystal, amethyst, citrine).</li><li>The chief constituent of sand and sandstone, and an important constituent of many other rocks.</li><li>It is piezoelectric and used to control the frequencies of radio transmitters.</li></ul>