Quartz is one of the most common minerals in the Earth's continental crust. It belongs to the hexagonal crystal system, and is made up of silica (SiO2) tetrahedra. Quartz has a hardness of 7 on the Mohs scale. Density is 2.65 g/cm³. The typical shape is a six-sided prism that ends in six-sided pyramids, although these are often twinned, distorted, or so massive that only part of the shape is apparent from a mined specimen. Additionally a bed is a common form, particularly for varieties such as amethyst, where the crystals grow up from a matrix and thus only one termination pyramid is present. A quartz geode consists of a hollow rock (usually with an approximately spherical shape) with a core lined with a bed of crystals.