Chertis a fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline silica.
Chalcedonyis a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic. Chalcedony's standard chemical structure (based on the chemical structure of quartz) is SiO2 (silicon dioxide).
Calming stone, especially for public speakers. Beneficial for lawyers, politicians and actors.