Quartz is a hard, crystalline mineral composed of silicon and oxygen atoms. The atoms are linked in a continuous framework of SiO₄ silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO₂. Quartz is the second most abundant mineral in Earth's continental crust, behind feldspar. Quartz crystal is valued for its piezoelectric and pyroelectric properties, by which it can transform mechanical pressure or heat into electromagnetic energy, and vice versa. Its ability to focus, amplify, store and transform energy is used throughout the technology world in ultrasound devices, watches, microphones, radio transmitters and receivers, memory chips in computers and other electronic circuitry.