SiO2. A hard, crystalline, transparent, often coloured mineral made of silicon dioxide. Quartz is one of the most common minerals in the earth's crust and occurs as grains (sand), masses (agate, bloodstone, chalcedony, jasper, carnelian, etc.) or crystals (rock crystal, amethyst, citrine, etc.). It can also be found as a major constituent of rocks such as granite. Quartz usually crystallises in hexagonal prisms or pyramids. It has been mined or gathered as a semiprecious stone since Paleolithic times. Sand is an additive in porcelain, brick, cement and mortar. Because of its hardness Mohs 7), quartz is also used as a cutting abrasive. Synonyms: sand; agate; bloodstone; chalcedony; jasper; carnelian; sard; rock crystal (colourless); amethyst (purple); citrine (yellow); onyx; rose quartz (pink); smoky quartz (brown to black); yellow quartz; milky quartz (milk white); chrysoprase. Museum of Fine Arts, Boston (2000); English Heritage (2000); Trench (2000). [J. Cassar, R. de Angelis]