Quartz

**Chemical name:** Silica **Molecular formula:** SiO₂ **Molar mass:** 60.08 g/mol **Mohs scale hardness:** 7 **Crystal system:** Lower quartz forms in the rhombohedral system, while higher quartz crystallizes in the hexagonal system. **Arrangement of atoms:** The SiO₄ tetrahedra are linked together at their vertices, forming a helical structural pattern. **Crystal habit:** Quartz can occur in either long or short columnar forms. **Description:** The color of quartz can vary significantly, including clear (crystal), yellow (citrine), pink (rose quartz), purple (amethyst) and brown (smoky), although it is typically white or gray. There are two polymorphs of silica: lower quartz, which is stable below 573 °C, and higher quartz, which is stable above 573 °C. **Applications:** Quartz is used in the production of glass and optical devices. Colored varieties of quartz are also valued as gemstones in jewelry making.