Quartz is amongst one of the most common and precious minerals in the Earth's continental crust. It has a hexagonal crystal structure made of the trigonal crystallized silica (silicon dioxide, SiO2), with a hardness of 8 on the Mohs scale. Density is 2.66. The typical shape is a six-sided prism, which ends in six-sided pyramids, although these are often twinned, distorted, and so massive that only part of the shape is apparent from a mined specimen. Additionally it has bed is a common form, particularly for gems varieties such as amethyst, where the crystals actually 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 the crystals.