How Light Becomes Color
The properties of an opal begin with a contradiction: the stone contains not a single pigment, yet it glows in every color. This play of color is pure physics, not pigment, and you can trace it all the way down to the size of tiny spheres.
A look deep inside the stone: how the light is diffracted within it, why of all things the size of the spheres decides the color, and how play of color and opalescence differ in physical terms.
What is an opal in physical terms?
An opal is a mineraloid made of hydrated silica, chemically SiO2·nH2O. Mineraloid means: it is a solid substance, but unlike any crystal it has no ordered atomic lattice. At the level of its atoms, opal is therefore disordered, almost like solidified glass.
What matters is the level above: inside the stone sit tiny spheres of exactly this silica. When they are stacked regularly, play of color arises.
Quartz, sapphire and diamond are crystals with a strictly ordered atomic lattice. Opal is not. Its colors do not arise in the atomic lattice, but in the larger structure of its spheres.
What is play of color, and why is it not a real color?
Play of color is a structural color. It is not created by a pigment that absorbs certain colors, but by the stone's microstructure, which redirects the light. You see exactly the same kind of color on a soap bubble, in the eye of a peacock feather, or on a butterfly wing.
Because no pigments are involved, the color is not fixed. It depends on how the light is diffracted inside, and it shifts as soon as you move the stone.
How does the sphere lattice diffract light?
Inside a precious opal, the silica spheres sit in a regular, three-dimensional lattice. In physical terms this is a photonic crystal: a structure that does to light exactly what an atomic lattice does to X-rays. It diffracts it.
When white light hits this lattice, a small part of it is reflected back at every layer of spheres. These many reflected waves overlap, and this is where interference comes in: when crest meets crest, the waves reinforce each other and the color flares up (constructive interference). When crest meets trough, they cancel each other out (destructive interference). That leaves a single wavelength, which exits the stone as a glowing color.
In detail, this happens in two steps. First, on entering, the light is refracted at the surface, because opal is optically denser than air. Inside, each layer of spheres then becomes a source of new light waves. That is precisely what Huygens's principle describes: the superposition of these many small waves produces the diffracted wave that you see as color.
How the light enters the stone
First refraction at the surface, then new waves at the spheres.
The next diagram shows how the individual colors fan out from this.
How the light fans out
White light hits the lattice, is diffracted at the layers and leaves the stone as spectral colors.
More precisely, the stone's refractive index n also comes into play. The full form reads λ = 2·d·√(n² − sin²θ). For an intuitive grasp this is enough: same angle, larger layer spacing, longer wavelength.
1. Diffraction
At each layer of spheres, part of the light is reflected back.
2. Interference
The waves overlap. In step they reinforce each other, out of step they cancel out.
3. Reinforcement
Matching waves reinforce each other into one color, the rest cancels out.
Researchers now recreate exactly this structure artificially. Synthetic opals are considered a textbook example of photonic crystals and are being studied for sensors and optical components.
Why does sphere size determine the color?
Which color is reinforced depends on the spacing of the layers, and this spacing depends directly on the size of the spheres. Small spheres diffract short-wavelength light and produce blue and violet. Large spheres diffract long-wavelength light and produce orange and red.
Short wave, long wave
From top to bottom the light wave gets longer, and the spheres needed get larger.
| Color | Spheres (approx.) | Light wave (approx.) | Frequency & value |
|---|---|---|---|
| Violet | ~150 nm | ~410 nm | rare, usually with blue |
| Blue | ~190 nm | ~470 nm | most common color |
| Green | ~220 nm | ~520 nm | common |
| Yellow | ~250 nm | ~580 nm | rarer, more valuable |
| Orange | ~280 nm | ~610 nm | rare |
| Red | ~300+ nm | ~680 nm | rarest, most valuable |
Orders of magnitude. The exact values shift depending on how the spheres are packed and on the stone's refractive index.
Red play of color requires the largest and most uniform spheres, and those form extremely rarely in nature. A stone with red fire usually shows all the shorter wavelengths alongside it, which often makes it the most colorful of all. Together, these two things make red the most valuable flash.
Why does the color shift when you move the stone?
Because the condition for reinforcement depends on the viewing angle. Tilt the stone and you change the angle θ at which the light meets the lattice. Suddenly a different wavelength satisfies the condition, and you see a different color. The ordered lattice stays the same; only your viewing angle shifts the result.
One structure, many colors
The same spot in the stone glows blue at one angle, green at another, red at a third. Exactly this shifting is what separates real play of color from a printed image.
Opalescence or play of color: what is the difference?
Both effects arise at the same spheres, but in opposite ways. Play of color means sharp spectral flashes and requires a strictly ordered lattice. Opalescence is the milky-bluish sheen and arises when the spheres are disordered or very small and merely scatter the light diffusely.
Play of color
Diffraction at the ordered lattice. Sharp, shifting spectral colors. The hallmark of precious opal.
Opalescence
Scattering at a disordered structure. Milky-bluish sheen without spectral flashes. Typical of common opal.
A common opal, often called potch, shows only opalescence. Only the ordered structure turns it into a precious opal with play of color.
Refraction and the other physical properties
Because opal is amorphous, it refracts light only singly, meaning equally in all directions (isotropic). And with a refractive index of 1.37 to 1.52 it has the lowest value among the common gemstones. Light is therefore bent comparatively little inside it.
Important: Refraction is not the cause of the play of color. A diamond's fire comes from dispersion, that is, from light being split as it is refracted. An opal's colors, by contrast, come from diffraction at the lattice. Two completely different mechanisms.
Hardness compared
| Property | Value | What it means |
|---|---|---|
| Class | Mineraloid (amorphous) | no ordered atomic lattice, unlike any crystal |
| Formula | SiO2·nH2O | hydrated silica |
| Mohs hardness | 5.5–6.5 | softer than quartz, so it needs protection |
| Density | 1.98–2.25 g/cm³ | light, because of its water and fine pores |
| Refractive index | 1.37–1.52 | lowest among the common gemstones |
| Refraction | singly refractive (isotropic) | a direct consequence of the amorphous structure |
| Source of color | Diffraction at the lattice | structural color, no pigment, no dispersion |
| Water content | 3–21%, usually 6–10% | makes the stone lively and delicate at the same time |
Why is opal so delicate?
The same two properties that make opal so beautiful also make it delicate: its water content and its low hardness. In strong heat or very dry air it can dry out and develop fine cracks, and with a hardness of 5.5 to 6.5 it is prone to scratches.
How to care for it properly is explained in our guide to opal care. Why exactly this physics decides the value of a stone is shown in the article on how opals are valued. And how an opal forms in the first place, over millions of years, is explained in the article on the formation of opals.
Real play of color, verified authenticity
Real play of color only occurs in real opal. Every piece of Liberty Angels jewelry is GSB-certified: genuine opal in 925 silver or 14K gold. Each certificate is linked to your order online, which makes it forgery-proof.
Frequently asked questions
Why does an opal shimmer in different colors?
Because an ordered lattice of tiny spheres diffracts the light and makes it overlap. The size of the spheres determines the color; the viewing angle makes it shift.
Is play of color a real color?
No. It is a structural color and comes from the stone's microstructure, not from pigment. The same principle is at work in soap bubbles and butterfly wings.
What is the difference between opalescence and play of color?
Play of color means sharp spectral flashes produced by diffraction at an ordered lattice. Opalescence is the milky-bluish sheen produced by diffuse scattering at a disordered structure.
Why is red opal so rare and expensive?
Red play of color requires the largest and most uniform spheres. Those form extremely rarely, which is why red is the most valuable color flash.
Does an opal have a high refractive index?
No, quite the opposite. At 1.37 to 1.52, opal has the lowest refractive index of the common gemstones. Its fire comes not from refraction but from diffraction.
Is the fire in an opal the same as in a diamond?
No. A diamond's fire comes from dispersion when light is refracted. An opal's colors come from diffraction at the sphere lattice. Two different physical effects.
