Quartz is a very common mineral, a chemical compound of silicon and oxygen, silicon dioxide SiO2, commonly called silica.Colored quartz varieties have been used for jewelry for ages, but most quartz is used as a component of concrete: quartz sand and quartz gravel. Uses : Pure quartz is needed for producing glass, ceramics, and chemical apparatus. Quartz glass, also known as "fused quartz" or "fused silica" (produced by quickly cooling molten quartz) has a number of interesting properties: its thermal expansion coefficient is very low, it is transparent for ultraviolet light, it is chemically almost inert, and it can form very thin but strong threads used in physical instruments. A well known application of quartz is its use as an oscillator in electric circuits in watches and computers. Less well known is, for example, its use as a membrane in ultrasonic devices.