Opal beauty: Octobers birthstone in a class all its own
The modern birthstone associated with the month of October is opal. Opal is one of the things that Australia is famous for, and that country has the largest known deposits of this amorphous mineral. Consequently, the Australians have adopted precious opal as their national gemstone.
Although most opal has a non-crystalline or amorphous structure that is somewhat similar to that of glass, some opals do exhibit localized micro-crystalline structures. Viewed through a microscope, regular amorphous opal consists of tiny spheres of silica that have been stacked together in layers, with water filling the spaces between the silica spheres. Generally, the greater the micro-crystallinity of an opal specimen, the more striking its appearance becomes and the more its value increases. Other minerals that find their way into the opal structure become impurities that provide additional physical properties (color variations, for example) to an opal.
Play of color
Opals come in a wide variety of colors and color patterns; but the most valuable types, the precious opals, display different shimmering rainbow-like color "flashes" or patterns when viewed from different angles. This striking visual display of "color play" is a type of iridescence which occurs as white light passes through the layer boundaries of the silica spheres and gets diffracted into the separate wavelengths of visible radiation that comprise white light. Also, the multiple structural layers redirect or reflect these diffracted wavelengths of light in such a way that they mix or interfere with one another. The same optical phenomenon can also be observed on the plumage of some birds, on insects wings, on some snail shells, on some pearls and agates, and on soap bubbles.
Opal stones typically contain about 10% water by weight. This water is chemically bonded to the silica spheres but the bonding is weak enough that excessive exposure to heat can cause this water to evaporate. Therefore opals need to be kept away from sources of heat to prevent them from dehydration. Excessive heat can alter the color or translucence of an opal gem and can even cause the gem to crack.
Numerous legends, both good and bad, have been associated with opals since ancient times; but it is the distinctive and delicate beauty of the opal that has made it desirable to many men and women throughout history. World-renowned opals include the Olympic Australis, the Andamooka Opal, the Flame Queen Opal, the Halley's Comet Opal, the Roebling Opal, and so on. Combined with other precious gems as accents and set in silver, gold, palladium or other metals, the opal is truly a beauty to behold.