A Gemstone Without a Crystal Structure

Almost every gemstone we cherish—diamonds, rubies, emeralds, quartz—derives its beauty from a highly organized, rigid crystalline lattice. Their atoms are stacked in perfect, repeating geometric patterns.

Opal is the great rebel of the gem world. It is an amorphous solid, meaning it has no crystalline structure whatsoever. Chemically, opal is incredibly simple: it is hydrated silicon dioxide (SiO₂·nH₂O). It is essentially just microscopic balls of silica gel that have hardened over millions of years, trapping anywhere from 3% to 21% water within its mass.

Yet, despite this chaotic, non-crystalline structure, precious opal produces the most organized, vibrant, and dynamic display of color in the natural world. To understand how a rock made of disorganized silica jelly creates flashes of fiery red and electric blue, we must look through an electron microscope.

1. The Microscopic Silica Spheres

For centuries, the source of opal's color was a mystery. It wasn't until the invention of the scanning electron microscope in the 1960s that scientists finally unlocked the secret.

When viewed at massive magnification, precious opal is not a solid mass. Instead, it is composed of billions of perfectly round, microscopic silica spheres, stacked together in a highly organized, three-dimensional grid, much like a box of tightly packed ping-pong balls.

The spaces in between these spheres are filled with water and a secondary silica solution that acts as a glue, holding the structure together.

2. The Physics of Play-of-Color

The phenomenon we see as flashing rainbows in a precious opal is technically known as play-of-color. This is entirely different from the optical effects seen in diamonds (dispersion) or moonstone (adularescence).

Play-of-color is caused by light diffraction.

How Diffraction Works

When white light enters the face of the opal, it passes through the translucent silica spheres. However, when the light hits the microscopic gaps and spaces between the spheres, the light waves are forced to bend and split (diffract).

Because the spheres are stacked in a perfectly uniform grid, the gaps between them act as a three-dimensional diffraction grating—the exact same physics principle that creates the rainbow reflections on the back of a CD or DVD. The white light is split into its component spectral colors (red, orange, yellow, green, blue, indigo, violet).

Sphere Size Dictates Color

The specific color that flashes back at your eye is dictated by a single factor: the exact diameter of the microscopic silica spheres.

  • Small Spheres (approx. 150 nanometers): The small gaps between these spheres only allow short-wavelength light to diffract. These opals will only flash Blue and Violet.
  • Medium Spheres (approx. 200 nanometers): These gaps allow mid-wavelength light to diffract, producing flashes of Green and Yellow.
  • Large Spheres (approx. 300 nanometers): These gaps are large enough to diffract the longest wavelengths of visible light, producing incredibly rare and valuable flashes of Red and Orange.

Because large spheres are much rarer in nature than small spheres, red-flashing opals are the most valuable, followed by orange, green, and finally blue.

3. Precious Opal vs. Common Opal (Potch)

If all opal is made of silica spheres, why don't all opals flash with rainbows?

The vast majority of opal mined from the earth is known as "Common Opal" or "Potch." When examined under a microscope, the silica spheres in potch are all different, random sizes, and they are jumbled together chaotically. Because there is no uniform grid and the gaps are inconsistent, the light is simply scattered in random directions rather than being cleanly diffracted.

This random scattering of light creates Opalescence—a milky, pearly, hazy glow, but no distinct colored flashes.

For precious opal to form, the groundwater carrying the silica must have remained perfectly undisturbed for thousands of years, allowing spheres of the exact same size to slowly settle into perfect alignment.

4. The Magic of Ethiopian Hydrophane

A fascinating sub-category of precious opal is the hydrophane opal, primarily sourced from the Welo region of Ethiopia.

Unlike solid Australian opals, Ethiopian opals are highly porous, almost like a rigid sponge. Because of these pores, they can absorb massive amounts of water. When an Ethiopian opal is submerged in water, the pores fill up, which drastically alters the stone's refractive index. The stone may turn completely glass-clear, and the play-of-color will vanish entirely!

Fortunately, this effect is temporary. Once the stone is removed from the water and allowed to air-dry over a few days, the water evaporates from the pores, the refractive index returns to normal, and the brilliant play-of-color magically reappears.

Understanding the physics of opal makes wearing one even more special. You aren't just wearing a stone; you are wearing a microscopic, three-dimensional light-bending machine created by nature.