Few gemstones can rival the visual drama of a fine black opal. Against a naturally dark background, flashes of red, orange, yellow, green, blue and violet can appear to glow from within the stone, creating a miniature rainbow that seems to move as the gem catches the light. For October babies, black opal represents one of the most spectacular expressions of the month’s official birthstone.
The finest examples of black opal come from Lightning Ridge, a remote outback mining region in New South Wales, Australia, where the gem has been mined since 1903. Black opals are distinguished by their dark gray to black body color, which provides a dramatic backdrop for their play-of-color. That contrast makes the colors appear particularly vivid, helping make top-quality Lightning Ridge black opals among the world’s most coveted opals.
Black opals are also exceptionally difficult to find. The dark-body material represents only a small fraction of the opal recovered at Lightning Ridge, and the stones with the most desirable combinations of intense color, strong red flashes and attractive patterns are rarer still.
Mining itself is challenging: the fields are remote and the opal-bearing layers can be buried beneath difficult outback terrain, requiring miners to sink shafts and work underground with specialized equipment. Decades of mining have also made the richest deposits increasingly difficult to reach. For independent miners, the considerable cost, physical demands and uncertainty of the work can make every promising pocket a high-stakes discovery.
The 26.9-carat black opal, shown above, was sourced at Lightning Ridge and gifted to the Smithsonian in 1970 by Mrs. Oliver B. James. Today, the cabochon-cut gem is a permanent part of the National Gem Collection at the Smithsonian Institution in Washington, DC. Its face displays an extraordinary spectrum of red, violet, blue, green and orange.
Scientists believe that between 100 million and 97 million years ago, an enormous inland sea covering much of Australia began to retreat. As the landscape dried, silica-rich material carried by water seeped into cracks and cavities in the underlying rock. Over immense stretches of time, the silica formed the hydrated silica mineral we know as opal.
Precious opal gets its spectacular play-of-color from microscopic silica spheres arranged in an orderly structure. When white light encounters those layers, it is diffracted into its component colors. The size and spacing of the spheres influence which colors appear, with the larger structures needed to produce the highly coveted red hue generally being rarer than those producing blue and green.
Much of the material recovered from opal fields is called “potch,” an opal without play-of-color. It contains the same basic silica composition as precious opal, but its microscopic spheres lack the orderly arrangement needed to create the rainbow effect.
Black opal is only one member of the opal family. White opal has a light body color, while boulder opal forms with a layer of ironstone attached to the opal. Crystal opal is transparent or translucent enough to allow its play-of-color to be seen through the body, and fire opal, best known from Mexico, typically displays a yellow, orange or red body color.
Australia remains the world's leading source of fine opal, with major fields beyond Lightning Ridge. Opal is also mined in Mexico, Ethiopia, Brazil, Honduras, the Czech Republic and parts of the United States, including Nevada and Idaho.
An opal contains 3% to 21% water within its silica structure, and its appearance and value depend on factors including body tone, brightness, color, pattern and consistency from different viewing angles.
Credit: Photo by Chip Clark/Smithsonian.