Cacholong
Cacholong also known as Kalmuck Agate is a form of common Opal although it is often mistaken for Agate or Chalcedony. A milky white colour with a Moh hardness of about 6, it is used for carving, cameos etc. Found in Austria, Czech Republic, Mongolia and Uzbekistan; its name possibly comes from a river in Uzbekistan.
Source: adapted by the editor from Wikipedia, the free encyclopedia; from the article "Cacholong". Image Credit.