In November 1986, a group of five adventurous miners in Australia embarked on a daring expedition up Lunatic Hill, a place aptly named for the seemingly irrational decision to dig there. While other miners were searching for opals just a few meters below the surface, this group, known as the Lunatic Hill Syndicate, decided to dig 20 meters deep. Their boldness paid off when they unearthed a remarkable opal, which they named the Halley’s Comet opal, inspired by the famous comet that was passing by Earth at the time.
The Halley’s Comet opal is extraordinary, yet its uniqueness is a common trait among opals. Unlike diamonds, rubies, or emeralds, which often look similar, each opal is distinct due to a phenomenon known as “play of color.” This captivating display of shifting colors is the result of a fascinating interplay of chemistry, geology, and optics that begins deep underground.
Opals start their journey as something quite ordinary: water. As water seeps through the earth, it travels through sandstone, limestone, and basalt, collecting a compound called silicon dioxide. This silica-rich water fills the voids in volcanic rocks, ancient riverbeds, wood, and even the bones of prehistoric creatures. Over time, the water evaporates, leaving behind a silica gel. Within this gel, millions of tiny silica spheres form in concentric layers, eventually hardening into a glass-like material with a lattice structure.
Most opals have a random structure, resulting in common opals with dull appearances. However, a small percentage, known as precious opals, have regions where silica spheres of uniform size create orderly patterns. This structure is key to their vibrant colors, explained by a principle of wave physics called interference.
Imagine a single color of light, such as green, with a wavelength of 500 nanometers, hitting a precious opal. The light scatters and reflects at different intensities depending on the angle. Some light reflects off the top layer, while other light reflects off deeper layers. When the distance traveled by the light matches a multiple of its wavelength, constructive interference occurs, amplifying the light and producing a brighter color.
By changing your viewing angle, you alter the distance light travels between layers, leading to different colors appearing. This is due to destructive interference, where the light waves cancel each other out. Different colors have different wavelengths, requiring specific silica bead sizes for constructive interference. For instance, blue light is amplified by beads around 210 nanometers, while red light, with its longer wavelength, needs beads closer to 300 nanometers. This makes red the rarest opal color.
The arrangement of silica spheres within an opal creates a variety of color patterns, from broad flashes to pin-fire and the extremely rare harlequin pattern. The conditions required to form precious opals are so rare that they occur in only a few places worldwide. Remarkably, about 95% of these opals come from Australia, where an ancient inland sea provided the perfect environment for their formation over 100 million years ago.
As we ponder the future, it’s intriguing to consider how silica-rich water might interact with the remnants of human civilization over the next 100 million years. What dazzling opalescent displays might emerge from the forgotten artifacts we leave behind in the darkness?
Engage in a hands-on simulation where you recreate the formation of opals. Use materials like gelatin and small beads to mimic the silica spheres and observe how light interacts with different structures. This will help you understand the process of opal formation and the science behind their unique colors.
Conduct an experiment to explore the concept of light interference. Use a laser pointer and a diffraction grating to observe how light waves interact. This activity will deepen your understanding of how opals produce their vibrant colors through constructive and destructive interference.
Participate in a workshop where you examine various opal samples and identify different color patterns. Learn to distinguish between broad flashes, pin-fire, and the rare harlequin pattern. This activity will enhance your ability to recognize and appreciate the diversity of opal patterns.
Join a field trip to a local museum or gemstone exhibit to see opals and other gemstones in person. Observe the play of color in opals and compare them with other gemstones. This real-world experience will reinforce your understanding of the unique characteristics of opals.
Write a short story or essay imagining the future of opals and how they might form from human artifacts. Consider the geological and chemical processes involved. This creative exercise will encourage you to think critically about the long-term impact of human civilization on natural formations.
On an auspicious day in November of 1986, five 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. 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 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 that 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?
Opal – A hydrated amorphous form of silica, often used in physics to study diffraction and interference due to its unique play of color. – The opal’s vibrant colors are a result of the diffraction of light within its silica structure.
Silica – A chemical compound composed of silicon and oxygen, commonly found in sand and used in the study of materials science. – Silica is a crucial component in the manufacturing of glass and optical fibers used in physics experiments.
Color – The characteristic of visual perception described through color categories, with physical properties such as wavelength and frequency. – The color of an object is determined by the wavelengths of light it reflects, absorbs, or transmits.
Light – Electromagnetic radiation within a certain portion of the electromagnetic spectrum, essential for various physics experiments and theories. – The speed of light in a vacuum is a fundamental constant in physics, denoted by the symbol ‘c’.
Interference – A phenomenon in which two waves superpose to form a resultant wave of greater, lower, or the same amplitude, often studied in optics. – The interference pattern observed in the double-slit experiment is a classic demonstration of the wave nature of light.
Chemistry – The branch of science concerned with the substances of which matter is composed, their properties, and reactions. – Understanding the chemistry of materials is essential for developing new technologies in physics.
Geology – The science that deals with the Earth’s physical structure and substance, its history, and the processes that act on it. – Geology provides insights into the Earth’s formation, which is crucial for understanding planetary physics.
Physics – The natural science that studies matter, its motion and behavior through space and time, and the related entities of energy and force. – Physics seeks to understand the fundamental principles governing the universe, from subatomic particles to galaxies.
Patterns – Regular and intelligible forms or sequences discernible in the natural world, often analyzed in physics to understand underlying laws. – The patterns of interference fringes can reveal information about the wavelength of light used in the experiment.
Water – A transparent, tasteless, odorless, and nearly colorless chemical substance, essential for all known forms of life and studied for its unique physical properties. – Water’s high specific heat capacity plays a significant role in climate physics and the regulation of Earth’s temperature.
