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**Chemical formula:** SiO₂ **Hardness:** 7 **Density:** 2.6 g/cm³ **Crystal system:** Trigonal (α), Hexagonal (β) **Luster:** Vitreous **Description:** Quartz is the second most abundant mineral in Earth's continental crust, after feldspar. It exists in many varieties, some of which are semi-precious gemstones, including transparent pure quartz, pale pink rose quartz, gray smoky quartz, pale yellow citrine, purple amethyst, black morion quartz, agate, onyx, heliotrope and jasper. Quartz is an essential rock-forming mineral with widespread geological significance. ## Keywords mineral quartz texture color ornamental stone semi-precious gemstone
