Boulder opal is a great alternative to the other two main types of opal: Black opal and White opal. Boulder opal is mostly sold in opal shops as a free form piece of gemstone. That means that the stone has its own individual shape rather than an oval or a circle or a pear shape which black and white opals are normally cut into.
A free form shape will not look like any shape that you recognize but will still be pleasing. Another major difference is that boulder opal will often not be smooth and domed on the top. We call this a cabochon from an old French word which translates to the English word hat. We cut most opals into cabochons because it enhances their color characteristics by allowing the light to enter and exit the stone for maximum effect.
Boulder opal will mostly have an undulating surface of the opal material sitting on a bed or dark brown ironstone. This means that the stone will need to have a jeweler make a special setting for the stone because ready made settings or castings generally come in oval shapes which are made for cabochon cuts of stones and not free form stones.
There are two sub types of boulder opal: seam and matrix. Seam opal is found where the opal material invades cracks in the ironstone rock. When the ironstone boulder is broken apart of by running a bulldozer across it or hitting it with a big hammer the rock will split into two parts. The opal will be in each part.
Matrix opal is where the opal runs in a haphazard fashion throughout the rock. So the actual stone is cut and polished with the opal matrix within it. It makes great opal pendants.
Boulder opal is heavier than black opal or white opal because it often has the ironstone with it so it makes it a little more difficult to make into some kinds of opal jewelry such as an opal ring or opal earrings. Certainly not impossible to make into anything but for pendants it is ideal and it makes up into large opal pendants as well.
Gary Hocking makes custom jewellery specializing in Australian opals. He has his own website http://www.opaljewelryexpress.com Feel free to use this article as long as you keep the bio and the live link to his website.