On an auspicious day in November of 1986, 5 Australian miners climbed Lunatic Hill— so named for the mental state anyone would be in to dig there. While their competitors searched for opals at a depth of 2 to 5 meters, the Lunatic Hill Syndicate bored 20 meters into the earth. And for their audacity, the earth rewarded them with a fist-sized, record breaking opal. They named it the Halley’s Comet opal, after the much larger rocky, icy body flying by the earth at that time.
The Halley’s Comet opal is a marvel, but its uniqueness is, paradoxically, the most usual thing about it. While diamonds, rubies, emeralds, and other precious stones are often indistinguishably similar, no two opals look the same, thanks to a characteristic called "play of color." This shimmering, dazzling, dancing display of light comes about from a confluence of chemistry, geology, and optics that define opals from their earliest moments, deep underground.
It’s there that an opal begins its life as something surprisingly abundant: water. Trickling down through gaps in soil and rock, water flows through sandstone, limestone, and basalt, picking up a microscopic compound called silicon dioxide. This silica-enriched water enters the voids inside pieces of volcanic rock, prehistoric river beds, wood and even the bones of ancient creatures. Gradually, the water starts to evaporate, and the silica-solution begins forming a gel, within which millions of silica spheres form layer by layer as a series of concentric shells. The gel ultimately hardens into a glass-like material, and the spheres settle into a lattice structure. The vast majority of the time, this structure is haphazard, resulting in common, or potch, opals with unremarkable exteriors. The tiny, mesmerizing percentage we call precious opals have regions where silica beads of uniform size form orderly arrays.
So why do those structures produce such vibrant displays? The answer lies in a principle of wave physics called interference. For the sake of simplicity, let’s look at what happens when a single color of light— green, with a wavelength of 500 nanometers— hits a precious opal. The green light will scatter throughout the gemstone and reflect back with varying intensities— from most angles suffused, from some entirely dimmed, and others dazzlingly bright. What’s happening is, some of the green light reflects off of the top layer. Some reflects off of the layer below that. And so on. When the additional distance it travels from one layer to the next, and back, is a multiple of the wavelength— such as 500 or 1000 extra nanometers— the crests and valleys of the waves match each other. This phenomenon is called constructive interference, and it amplifies the wave, producing a brighter color. So if you position your eye at the correct angle, the green light reflecting from many layers adds together. Shift the angle just a bit, and you change the distance the light travels between layers. Change it enough, and you’ll reach a point where the crests match the valleys, making the waves cancel each other out— that’s destructive interference.
Different colors have different wavelengths, which translates to varying distances they have to travel to constructively interfere. That’s why colors roughly correspond to silica bead sizes. The spaces between 210 nanometer beads are just right to amplify blue light. For red light, with its long wavelengths, the silica beads must be close to 300 nanometers. Those take a very long time to form, and because of that, red is the rarest opal color. The differences in the arrangements of the gel lattices within a particular stone result in a wide range of color patterns— everything from broad flash to pin-fire to the ultra-rare harlequin.
The circumstances that lead to the formation of precious opal are so uncommon that they only occur in a handful of places. About 95% come from Australia, where an ancient inland sea created the perfect conditions. It was there that the Halley’s Comet opal formed some 100 million years ago. Which raises the question: in the next 100 million years, silica-rich water will percolate through the nooks and crannies of some of the discarded artifacts of human civilization. What opalescent plays of light will one day radiate from the things we forget in the darkness?
