Boulder  Opal  is so named because it is found in Ironstone "geodes"
            or  "Boulders".   Small  "Boulders"  are sometimes called "Yowie
            Nuts".   The Opal frequently has some of the ironstone still in
            the colour, and almost always on the back.  Some Boulder Opal
            is  mostly  ironstone,  with  fine  lines  of  brilliant colour
            running  through  it.   This  type  is sometimes called "Matrix
            Opal",  which  should  not  be confused with Western Australian
            Matrix.  Western australian matrix does not exhibit it's colour
            until  it  is  etched and boiled in sugar water, after which it
            becomes quite pretty.