Silicon dioxide, also known as silica, is a chemical compound, the oxide of silicon. Its chemical symbol is SiO2. It is known for its hardness since the 16th century. It is an important compound used in most types of glass and other hard substances such as concrete, porcelain and stoneware.
Silicon dioxide exists in nature in a number of forms, including flint, quartz and opal. If it is breathed in as dust, it can cause silicosis and cancer, as the dust is trapped in the lungs. Besides the dangers of inhaling it in dust form, it is harmless.