The most colorful gemstones on Earth Jeff Dekofsky
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?