With the world’s primary supply of black and boulder opal coming from its wild outback, Australia is synonymous with opal. Since the first recorded discovery of precious opal in the 1840s, its fabulous play-of-color has delivered excitement and delight to consumers all over the world.
Australian opal is predominantly found in sedimentary rocks within the Great Artesian Basin (GAB). The Great Artesian Basin is the largest artesian basin on this planet and the main “water tank” that supplies inland Australia. The basin is the remnant of an inland sea filled with sediments. Some volcanic opals have been recovered, but production is very limited compared to sedimentary opal. Although the chemistry and the microscopic structure of opal have been well analyzed, the formation of this gemstone is still much debated within the research field.
Multiple models have been published with supporting evidence to demonstrate how opal formed. One certainty is that not all opal formed in the same way. The supply and transportation of silica, space for growth, sedimentation conditions, and ion exchange processes are some of the critical issues that must be settled before any definitive model can be constructed.
Opal is an amorphous form of silica, and precious opal shows the rainbow colors we call play-of-color, caused by the diffraction of light. Precious opal is rare as its structure is composed of tiny silica spheres tightly packed in a three-dimensional grid. Light is broken up into the colors of the rainbow by the tiny voids between the spheres. To give an idea of the size of the spheres, those that are 0.20 microns in size have spaces between them that diffract the light to cause a blue play-of-color; spheres of 0.25 microns give a green color; and spheres of 0.32 microns, a red color. These tiny spheres that are responsible for play-of-color require an electron microscope of at least 30,000X magnification to see them.
Black opal refers to opals that have a dark body color when viewed from above. They can be almost transparent to opaque, with an endless variety of play-of-color. Black opal is considered the rarest and most valuable opal. Boulder opal has thin veins of precious opal in the host rock the opal formed in, which is usually ironstone. The opal forms as infillings in the cracks and spaces of the ironstone. Boulder opal is usually cut free-form and wavy to follow the contours of the opal.
Add to the appearance equation the pattern of the play-of-color, the underlying body color, the brightness of the rainbow colors, the number of colors, the dominant rainbow colors, the thickness of the seam of play-of-color or color bar, the depth of the transparency, and the shape of the stone, and it is easy to see why each opal is unique and why opal is considered by many gem experts to be the most complex stone to grade. While market preferences determine the values of different opal appearances, opal is truly a personal stone with a beauty that is very subjective.