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. Wal.