Quartz is a tectosilicate mineral that is the second most common mineral in Earth's continental crust. Its crystal structure is a framework of SiO4 silicon–oxygen tetrahedra. Each shares an oxygen atom oxygen with another tetrahedron, so the overall chemical formula is SiO2 or silica.
There are many different varieties of quartz, some of which are semi-precious gemstones. They have been used for a long time to make jewelry and hardstone carvings. Agate, amethyst, rose quartz are all forms of quartz.
'Fused quartz' is glass made of silica in non-crystalline form. It does not contain other ingredients that are added to other glass to lower the melting point. Fused silica has high working and melting temperatures. For some purposes, fused quartz is better than other types of glass due to its purity. The glass is used in a number of high-tech products.