Opal is one of two birthstones for October (along with tourmaline). Most opals are valued for their shifting colors in rainbow hues which is known as a “play-of-color" when an opal is viewed from different angles. This flash of color or fire is caused by diffraction when light travels through tiny silicate spheres and edges within the opal. Like a prism, the opal splits the white light into all the colors of the spectrum resulting in an eyeful of beautiful opal. Since the interior structure of each opal is different, each opal is unique and individual. No two opals are alike.
Although opals are found in many places around the world, Australia is the most productive source for opals. Australian black opals are mostly mined in Lightning Ridge, in northern New South Wales. Black opal has a dark body tone which can range from dark grey to jet black. Because of their dark body tone, the rainbow colors in a black opal stand out distinctly.
Boulder opals are found only in the state of Queensland, Australia. Boulder opals are easily distinguished by their layer of solid brown ironstone left on the back of the stone. These opals are mined from large ironstone boulders under the ground. Due to the dark backing provided by the ironstone, boulder opals generally have a dark body tone which leads to a vibrancy of color similar to that found in black opals. Boulder opals are almost always cut in a free form shape.
White opal is found in the White Cliffs area of New South Wales, as well as in Mintabie, Andamooka and Coober Pedy in South Australia. Ethopia is a source of many different color opals as well as the crystal opal. The term “crystal opal” refers to any kind of opal which has a transparent, translucent, or semi-translucent body. where light is able to pass through the stone, or you can see through an opal.
Mexican fire opals are translucent to transparent with a yellow, orange, or red body color. The “fire” in their name refers to their body color, not to play of color. These stones are also called sun opals.
Opals have a hardness of 5 to 6.5 on the Moh's scale and can be scratched by other gems. Since opal is a soft stone, approximately the same hardness as glass, it is important to treat your opal carefully in order to avoid damaging it.
Opals are often stabilized to make them more durable and wearable. Gem cutters can take opal pieces too thin to use as a solid gemstones and assemble them into doublets and triplets. A doublet consists of a thin layer of precious opal glued to a black base. A triplet adds a transparent, quartz cap. Triplets make good ring stones because the hard quartz keeps the softer opal from scratching. Opal may be treated by impregnation with oil, wax or plastic.