Bukovskyite

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.