Quartz is the most common crystalline form of silica dioxide (SiO₂) and the second most abundant mineral in the Earth’s crust after feldspars. Quartz is a common rock-forming minerals that occurs in most acid and intermediate igneous rocks, in many metamorphic rocks, and in terrigenous sedimentary rocks. The name ‘quartz’ likely derives from the Saxon word ‘Querkluftertz’ (cross-vein ore), since it is also a common mineral in veins and metasomatic rocks.

Structure and chemistry
Quartz is a framework silicate and one of the naturally occurring polymorphs of SiO₂. There are two forms of quartz: α-quartz, stable below 573 °C at ambient pressure, and β-quartz, stable from 573 to 870 °C. β-quartz has an hexagonal crystal lattice with ‘rings’ of 6 SiO₄ tetrahedrons linked to each other. The structure of α-quartz is trigonal and characterized by a slight rotation of the SiO₄ tetrahedrons with respect to the β-quartz structure. The α-β transformation, indeed, involves no atom exchange or breaking of bonds and occurs very easily in nature. α-quartz is a chiral substance with crystal class 32 and, therefore, can exist in two forms: right-handed quartz and left-handed quartz. Right-handed quartz has a virtual helix of SiO₄ tetrahedrons that rotates clockwise with respect to the long-axis (c-axis). In left-handed quartz this rotation is anticlockwise. Quartz is commonly almost purely SiO₂, but it may accommodate some cations in traces like Al³⁺, Na⁺, Li⁺, K⁺, Ti⁴⁺, and Fe³⁺. In some cases, the impurity of quartz linked to these cations can produce colored varieties of quartz (e.g. smoky, rose, amethyst…). Quartz is commonly twinned. Some common twins are Dauphiné and Brazil twins, occurring between right-handed and left-handed crystals: the two individuals are generally penetrated with each other with their c-axis parallel to one another. Consequently, these twins are optically undetectable in standard thin sections. The Japan law is a twinning between two quartz crystals of the same type (left- or right-handed) where the two crystals are twinned on the prism faces and their c-axes meet at angle of 84°33’.