In November of 1986, 5 Australian miners climbed Lunatic hill (“Lunatic hill” sounds like the name of a bad horror movie). The Lunatic hill syndicate board 20 meters into the earth and for their audacity the earth rewarded them with a fist-sized record-breaking Opal. They named it Haley’s Comet opal. The Haley’s Comet opal is a marvel but its uniqueness is paradoxical, the most unusual thing about it. While diamonds, rubies, emeralds, and other precious stones are often indistinguishable a similar no two opals look the same due to their characteristic “cold play of color“.
The shimmering, dazzling, dancing display of light comes about from a convergence of chemistry, geology, and optics that defines opals from the earliest moments, deep underground. It’s there that’s an Opel begins its life as something surprisingly abundant.
Haley’s Comet Opal Formation
The water that trickles down through gaps in soil and rock, that 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 rocks, prehistoric riverbeds, and even the bones of ancient creatures and it starts to evaporate and the silica solution begins forming a gel within which millions of silica spheres formed layer by layer as a series of concentric shells. This gel ultimately hardens into a glass-like material and the spheres settle into a lattice structure. Mostly the structure formed is haphazard, resulting in common or potch opals with unremarkable exteriors. The tiny mesmerizing percentage we called precious Opals.
Why do those structures produce such vibrant displays
The answer is in the physics of the wave known as Interference. For the sake of simplicity let’s look at what happens when a single color of green light with a wavelength of 500 nanometers hits a precious opal, the green light scattered throughout the gemstone and reflect back with varying intensities. From most angles suffused from some entirely dimmed and others dazzlingly bright. What’s happens is that some of the green light reflects off of the top layer of the opal and some reflects off of the layer below that. 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 waves producing a brighter color.
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