Why Does Opal, a Stone, Shine in Rainbow Colours?
― How Tiny, Invisible Rows of Spheres Play Tricks With Light
Have you ever seen an opal in a grandmother's ring, or in a jeweller's display case? Diamonds and rubies sparkle, but their colour stays the same. An opal is different. Tilt the ring a little and the colours drift from blue to green to orange, as if a rainbow were trapped inside the stone.
In a jeweller's display case, gently tilt an opal ring. A patch that looked blue-green a moment ago turns orange or red as soon as you change the angle. It is one stone, yet its colour seems to shift depending on where you look from.
The odd part is that opal contains no dye of any particular colour. There is no pigment, yet the colours keep changing. How does that work?
The rainbow needs two things working together
Inside a rainbow-shimmering opal, glass spheres a few hundred nanometres across, far too small to see, are thought to be stacked in a regular pattern.
When light strikes the regular layers of spheres, the light reflected from each layer overlaps. Only the colour that fits the spacing between spheres is strengthened. The other colours cancel out and vanish.
Only when both are in place do we get the effect of "colour without pigment". Let's take them one at a time.
Reason 1: Invisible "glass spheres" inside the stone
The silicon dioxide that makes up opal is not a crystal. It is an amorphous material, like glass. "Amorphous" means the atoms are not lined up in a regular pattern the way they are in a crystal.
Inside opal, though, this amorphous glass forms tiny spheres. Their diameter is roughly 150 to 400 nanometres (one nanometre is a billionth of a metre), and they are thought to be nearly all the same size. The spheres are stacked neatly, with no gaps. Over a very long time, water carrying dissolved silica (silicon dioxide) is thought to have seeped through, slowly growing the silica into evenly sized spheres.
Opal with spheres arranged this regularly is called precious opal (opal that shows play of colour). Opal whose spheres vary in size or arrangement is called common opal, and it does not shine in rainbow colours.
Reason 2: Light "interference" caused by the sphere layers
When light hits the regular layers of spheres, the light reflected from each layer overlaps. Only colours whose wave peaks meet peaks, and troughs meet troughs, are strengthened (this is called interference). The other colours are out of step and cancel out. Which colour is strengthened depends on the sphere spacing and on the angle at which the light arrives.
If you change the viewing angle, the colour that meets the condition changes. That is why an opal's colours shift as you tilt it. In the gem world this effect is called play of colour.
This idea, that "regular spacing makes colour", is found in living things too. In our article on how chameleons change colour, we look at how a chameleon changes the spacing of crystals in its skin to control the colour of the light it reflects.
In common opal, the spheres are thought to vary in size and arrangement. When the spacing differs from sphere to sphere, the reflected colour differs too, so no single colour can be strengthened together. The colours cancel out, leaving a milky or opaque look, and no play of colour appears.
The same trick shows up elsewhere
- Morpho butterfly wings ― They have no blue pigment. Tiny structures on the scales strengthen blue light, and that gives the vivid blue.
- Peacock feathers ― Microscopic structures inside the feathers strengthen light, producing colours that change with viewing angle, an effect close to play of colour.
- The back of a CD or DVD ― Countless fine grooves cut into the surface split light by wavelength as they reflect it, so you see rainbow colours.
- Soap bubbles ― Light reflected from the front and back of a thin film interferes to make colour, as explained in this article. The way the interference happens is a little different, but "colour without pigment" is the same as in opal.
Something you can try at home
- Take an old CD or DVD and hold its recording side (the silver-looking side) up to a fluorescent lamp or sunlight
- Change the angle as you look, and rainbow-like bands of colour appear to move with it
The disc has extremely fine grooves, too small to see, cut in a regular spiral. This pattern of grooves sends each wavelength of light off in a different direction. The fine details differ from the spheres in opal, but "a regular, fine structure makes colour without any pigment" is true of both.
Summary
Opal shines in rainbow colours because of two things working together: (1) invisible glass spheres are stacked in a regular pattern, and (2) light striking that pattern is strengthened in certain colours only and sent back to your eye.
The colour of an opal is not the colour of the stone itself.
It is made fresh, right in front of the viewer's eyes, by light and a pattern too small to see.
For those who want to know more ― terms, formulas and links to textbooksFrom middle-school science to active research, each part is labelled with its level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school Basic Physics
- High school+High-school Physics, or extension and sidebar material in textbooks
- UniversityUniversity specialist subjects (crystallography, solid-state physics) not taught in high school
- ResearchTopics researchers are still investigating, not yet taught as settled fact even at university
Middle schoolTerms: words around opal
- Interference: When several waves overlap and strengthen or weaken one another.
- Amorphous: A state in which atoms are not lined up in a regular pattern like a crystal. Glass is thought to be in the same state.
- Play of colour: The name for the effect, special to opal, in which colours shift depending on the viewing angle.
- Silica: Silicon dioxide (SiO₂). The main component of opal and glass.
- Precious opal / common opal: Opal that shows play of colour, and opal that does not.
Middle schoolHigh schoolCheck it with a formula: how does sphere size change the colour?
Hearing that "the sphere spacing decides the colour" doesn't tell you how big the difference is. A calculation gives clear numbers.
Reinforced wavelength ≒ 2 × sphere spacing × refractive index of the surroundings
| Reinforced wavelength | The quantity that sets the colour you see [nm] |
| Sphere spacing | Roughly the diameter of a silica sphere [nm] |
| Refractive index of the surroundings | Taken to be around 1.4 for opal |
This is a simple approximation for looking straight on. In reality the angle matters too, but the formula does capture that the larger the spheres, the longer the reinforced wavelength.
| When the spacing is large | 2 × 250 × 1.4 ＝ 700 [nm] |
| When the spacing is small | 2 × 180 × 1.4 ＝ 504 [nm] |
Visible light covers roughly 400 (violet) to 700 (red) nanometres. 700 nm is near the red end, and 504 nm is green. A difference of only a few tens of nanometres in sphere spacing shifts the colour you see from red to green.
250 nanometres is only about 1/300 of the thickness of a human hair (roughly 0.08 millimetres). A tiny difference in the size of spheres too small to see decides the colour of the whole gemstone.
High school+UniversityWhy the colour changes with angle (more precisely)
The formula above was a simple approximation for looking straight on. In reality the viewing angle θ matters too. More precisely, it is handled with the relation mλ = 2d√(n² − sin²θ) (Bragg's diffraction condition applied to the sphere arrangement). Here m is the order of the reinforcing interference (usually 1), and n is the effective refractive index, including the material filling the gaps between spheres. When the angle θ changes, the right-hand side changes, so the reinforced wavelength λ changes as well. This is what lies behind play of colour, the colour shift you see when you tilt the stone.
Electron-microscope observations suggest that this regular arrangement of spheres is stacked in a way close to a face-centred cubic lattice. A material with a repeating structure on the scale of the wavelength of light, which strongly reflects or transmits only certain wavelengths, is called a photonic crystal, and opal is thought to be a leading natural example.
ResearchWhat we still don't fully understand
- How such evenly sized silica spheres come to stack so regularly in nature is not fully understood. Exactly how dissolved silica grows into uniform spheres and lines up over tens of thousands of years or more is still a research topic in mineralogy.
- Synthetic opal can reproduce a play of colour close to that of natural opal, but reliably making it as uniform in structure and as durable as natural opal is still considered difficult.
- Natural opal contains water, and it is known to develop cracks called "crazing" when it dries out. But the exact conditions, such as how much change in water content starts the cracking, have not yet been well quantified.
- Regular fine structures like opal's have influenced research aimed at pigment-free colour materials and sensors, but technology to mass-produce structures as uniform as natural opal is still being developed.
Even the beautiful colours seen in a jeweller's shop are still being studied down to the details of how they form. Being familiar is not the same as being understood.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: properties of light (reflection, refraction) | Arrangement of spheres; precious vs. common opal |
| High school | Basic Physics: properties of waves | Basic idea of interference; wavelength and colour calculation |
| High school+ | Physics: interference and diffraction of waves (advanced) | Colour change with angle; Bragg's diffraction condition |
| University | Solid-state physics and crystallography (photonic crystals) | Face-centred cubic lattice; photonic crystals |
| Research | Mineralogy and materials science (unsolved) | Self-assembly of silica spheres; limits of synthetic opal; mechanism of crazing |
| ― | Everyday examples | Comparison with CDs/DVDs, Morpho butterflies and soap bubbles |
- Sanders, J.V. (1964), Colour of Precious Opal, Nature 204, 1151–1153 (elucidating opal structure by electron microscope).
- Jones, J.B., Sanders, J.V. & Segnit, E.R. (1964), Structure of Opal, Nature 204, 990–991.
- Explanatory materials on opal structure and play of colour from the Gemological Institute of America (GIA) and similar bodies.
- General descriptions of interference, diffraction and the Bragg condition in optics textbooks.
- Materials-science explanatory materials on photonic crystals (research on artificial opal and applications of structural colour).
* Figures such as sphere diameter and refractive index vary between individual stones and between sources. This article gives commonly used rough values.
* This article is a general-audience science explainer. Figures on opal structure and optics vary between sources and are given as rough guides to help you understand the mechanism.