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Commonly known as silica, with the chemical formula SiO2. It is the most common and stable compound of silicon; it is widely found in nature, forming various minerals and rocks. Silica exists in both crystalline and amorphous forms; the most important form of crystalline silica is quartz, which is an atomic crystal. Pure quartz is colorless and transparent, with a glassy luster; it is hard in texture and is known as crystal. Crystals that exhibit different colors due to the presence of trace impurities are known as crystals. Those containing manganese dioxide are called sapphires; those containing carbon are known as onyx; those with bubbles in them are called milky crystals. In addition, there are also yellow crystals, amethysts, smoky quartzes, and so on – all of which are precious minerals. Common sand is silica mixed with a large amount of impurities. In nature, there is also amorphous SiO2, known as diatomite, which is commonly used as an adsorbent for glycerin** (nitroglycerin). SiO2 has very stable chemical properties – it is insoluble in water and in ordinary acids – but it can react with hydrofluoric acid to produce silicon tetrafluoride, SiF4, as a gas. When SiO2 is fused at high temperatures with strong bases or certain metal carbonates, silicates are formed. SiO2 is melted at high temperatures (with a melting point of about 1,600°C), and then the melt is allowed to cool. As the temperature drops, the melt becomes increasingly viscous until it finally hardens, resulting in quartz glass. Containers made of quartz glass do not break easily when exposed to sudden changes in temperature, and they allow ultraviolet light to pass through. Therefore, quartz glass is used to manufacture mercury vapor ultraviolet lamps as well as certain optical and high-temperature instruments that utilize ultraviolet light. Diatomite is used as an insulating and soundproofing material. Ordinary sand is a raw material used in the production of silicate products such as glass, ceramics, cement, and refractory materials.