October has two colorful birthstones: Tourmaline and Opal.
There are two types of Opal: precious or common, but only the precious Opals display iridescence, sometimes called “play of color.” Though they do not feature iridescence, common opals come in a wide range of rich colors and are widely used in the creation of gemstone jewelry.
The play of color optical effect occurs when stones are turned in white light and their layered internal structure causes diffraction of the light, resulting in the play-of-color phenomenon. The background color of Opal may be white, black or almost any other color of the visual spectrum. Black Opals are the rarest, and white, gray or green stones are the most common.
Opal is amorphous silica with a water content ranging from three to about twenty percent by weight, however, most stones typically contain between six and ten percent water. Because of its amorphous nature, Opal is classed as a mineraloid – unlike crystalline forms of silica, which are classed as minerals.
Depending on how it formed, opal may be opaque, translucent or transparent and is deposited at relatively low temperatures. Opal may occur in fissures of almost any rock but is typically found with Sandstone, Rhyolite, Marl, Limonite, and Basalt.
Metaphysical properties of Opal
The ancient Greeks believed that opal bestowed the gift of prophecy and gave the owner protection from disease. The Bedouins once believed that Opals fell from the sky during thunderstorms, and their play of color was lightning trapped within each stone.
Opal is believed to be a symbol of purity, hope and truth, and is also the stone given to celebrate the 14th wedding anniversary. It is the national gemstone of Australia.
Boulder Opal 
Opal cabochons from Ethiopia Ethiopian Opal cutting rough Mexican Fire Opal 
Variety of Australian Opal cabochons 
Boulder Opal box with Opal cabochons
On the market
Ethiopia, Mexico and Brazil are important Opal sources along with Europe, Honduras, Indonesia, Madagascar, Peru, Turkey and the United States.
Australia’s Opal fields are the most productive in the world. Spectacular Black Opal is found in Lightning Ridge, New South Wales, Australia, and is mined in a remote, arid and hot environment that is famous among gemstone lovers for the highly desirable but rare stones found there. White Opal from the White Cliffs region of New South Wales, along with Andamooka, Coober Pedy and Mintabie, are four other major Australian Opal producing regions. Captivating Boulder Opal with flashes of brilliant green and blue veins in a rich brown matrix is a material found in only one location in the world, and is only mined in Queensland, Australia.
Opal Treatments
Many Opals are treated by impregnation with oil, wax, or plastic, and by surface modifications to create more durable, easily-set gemstones. Thin layers of Opal with good play of color are often mounted in doublets or triplets to allow for an extra durable finished stone.
Opal Gemstones and Jewelry
Opals are suitable for jewelry but require special care when worn to avoid scratching, chipping or breaking them. Clean finished jewelry and stones gently with a soft cloth and warm, soapy water. Many Opal owners store their stones in clean water when they aren’t being worn to prevent them from drying out, especially in arid climates.
