Quartz is a major silicate mineral that consists of a series of silicon-oxygen tetrahedra in which each oxygen atom is shared, resulting in the chemical formula SiO2. It is the second most abundant mineral in Earth’s continental crust and it is relatively hard (Slavic ancestors of the German word quarz translate to “hard” in English). Quartz is often colorless but there are many different varieties in different colors, including semi-precious gemstones such as amethyst, citrine and chalcedony.
Information source: Mindat.org
Image: Quartz crystals found in the USA. Credit: Parent Géry, Wikimedia Commons.