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**Chemical name:** Silica
**Formula:** SiO₂
**Molar mass:** 60,08 g/mol
**Mohs scale hardness:** 7
**Crystal system:** Lower quartz rhombohedral system, higher quartz hexagonal system.
**Arrangement of atoms:** Tetrahedra SiO₄ are connected at their tops into the system helices.
**Crystal habit:** Long or short columnar.
**Information:** Colour quartz is very variable, eg.: Clear (crystal), yellow (citrine), pink (rose quartz), purple (amethyst), brown (smoky), etc.; mostly white but possibly grey. There are two modifications: Lower (stable below 573 °C) and higher silica (at a temperature above 573 °C).
**Use:** Quartz is used for the production of glass and optical devices. Coloured varieties are used in jewellery as gemstones.
