Queensland Boulder Opal is usually composed of thin sheets or veins of precious opal that has over millions of years seeped into the natural cracks or fissures in the native Ironstone.  The Ironstone with its dark color then becomes the natural backing for the opal brightening up the opals colors much in the same way that black potch does for black opal.  With boulder opal you can end up with a natural black opal effect and depending on how much of the face of the stone is covered with colorful opal it can also although rare  reach black opal prices.  Boulder opal should not be sold by carat weight because the ironstone weighs much more than opal and usually makes up most of the gem.  It should be instead judged by how much of the face of the stone is showing brite opal colors with the top tier being reserved for gems that show  about 90% opal and above with little ironstone showing through.