Quartz is a crystalline form of silica. The greatest concentrations of quartz are found in silica-rich igneous and metamorphic rocks. Quartz is one of Earth’s most abundant minerals and is an essential component of granite and many other rocks. Often transparent, Quartz can be found in almost any color. There are two main types of Quartz: macrocrystalline and microcrystalline. Macrocrystalline Quartz is composed of visible crystals, microcrystalline Quartz is densely compacted microscopic quartz crystals. Quartz has piezoelectric properties, meaning it generates an electric charge in response to pressure.
Chemical composition: SiO2 ; Hardness: 7
When there is a structure composed of another mineral within a crystal it is referred to as a phantom. Phantoms are formed when another mineral drops onto a crystal during its growth. The crystal continues to grow, encompassing the foreign mineral which appears in the shape of the crystal during an earlier stage of its growth.