1) Iron mineral 
Bukovskyite (also known as `clay of Kutná Hora`) is an iron arsenate sulfate mineral with formula: Fe2(AsO4)(SO4)(OH)·7H2O which forms nodules with a reniform (kidney-shaped) surface. Under a microscope, these nodules appear as a collection of minute needles similar to gypsum. Some can be seen with the naked eye and occur inside the nodules. B.....