This is a photograph of boulder opal still inside of it's host ironstone matrix.
Quite a pretty color, and you can see that opal, formed when molten lava liquifies sand into a glass like mixture and mixes with just the right amount of water (usually 3% to 10% on most gem quality but as high as 20% on stones found under the water table which have a high propensity to crack or craze) will flow through cracks in the ironstone (as pictured) until it can pool together and cool down forming the opal that you see here.
This stone was taken from "Coober Pedy" in Australia where most of the worlds commercial quality opal comes from. The quality of the opal in this stone would not warrant it to be cut into gem stones.
A "knobbie" is just a little pocket of black opal found in the ground usually the size of a small rock they are at complete random patterns scattered throughout the Lightning Ridge area of Australia. Anywhere the silica collects to form the opal.
The miners will dig into the ground until they find, what they call "Opal dirt" which is a Talc powder like substance which will usually mean opal is nearby. They will then follow this opal dirt horizontally through the dirt and see if they can find these little "knobbies"
Opal mining is done primarily by two man teams. There arent any big corporations like DeBeers doing any mining in Australia because the government there will only give any one individual 2 claims (50 meter x 50 meter sections) at a time. I've heard that these claims cost about 50 dollars each. There are quite a few hard luck stories though, for those that want to plunk down the $50 dollars and start digging opals for a living
thanks for the great pics! I love seeing stones in the rough like that, and that little "knobbie" is so pretty! I want one! How much does something like either one of those stones cost? rough idea...I have heard that black opal is really expensive.