- noun a mineral form of silica, often found as crystals in igneous rocks
- A mineral whose chemical formula is SiO2. It is the most commonly occurring mineral in the earth's crust. It is a crystalline form of silicon dioxide, has a hardness of 7 on the Mohs scale, and when pure it is a colorless and called rock crystal, although there are many colored varieties. Quartz has many applications, including its use in oscillators, frequency standards, clocks, piezoelectric devices, electronic components, and in optical instruments.