Why Do Opals Flash Color? The Physics of Play of Color

Opals flash color because they are built from microscopic spheres of silica stacked in a regular grid, and that grid splits white light into its colors the way a prism or a soap bubble does. The size of the spheres decides which colors appear: smaller spheres give blue and violet, larger ones give orange and red. October is opal's birthstone month, so for anyone in Los Angeles celebrating an October birthday, here is the physics behind the gem that never shows the same color twice.

What is an opal made of?

Most gems are crystals, with atoms locked in a repeating pattern. Opal is different. The Australian Museum describes it as a hardened gel of silica and water, usually 6 to 10% water, and notes that opal is amorphous, with no definite crystal structure. GIA gives its chemistry as SiO₂·nH₂O and says opal can contain up to 20% water trapped in its silica structure. Either way, there is water inside the stone, and that fact matters for how you wear it.

According to GIA, opal forms when silica gel fills crevices in rock. As the water slowly evaporates, the silica is deposited as tiny spheres.

Why does an opal show a rainbow of colors?

In precious opal those silica spheres are all nearly the same size and packed in a neat three dimensional grid, with tiny voids between them. The Australian Museum calls this structure a three dimensional diffraction grating. GIA uses a lovely image: silica spheres stacked like tiny Ping-Pong balls in a box.

When white light enters the grid, each layer of spheres scatters a little of it. The scattered waves overlap. For one particular wavelength, the waves line up crest to crest and reinforce each other. For other wavelengths, they cancel. The result is a flash of one pure color coming from that patch of the stone. Tilt the opal and the angle changes, so a different wavelength is reinforced and the color shifts. Gemologists call this play of color.

• Diffraction: light bends and spreads as it passes a regular structure.
• Interference: overlapping light waves add up or cancel out.
• Angle: changing your viewing angle changes which color wins.

How does sphere size decide the color?

The spacing of the grid has to be close to the wavelength of light for the effect to work, and visible light has wavelengths of a few hundred nanometers. The Australian Museum gives the numbers: when the spheres reach about 138 nanometers in diameter, they can diffract violet, the shortest visible wavelength. Spheres over 241 nanometers can produce red. GIA puts it more simply: grids of spheres around 0.2 microns create red flashes.

That is why an opal full of red and orange fire is rarer than one that shows only blue and green: it needs larger spheres, stacked just as neatly. And when the spheres are too large, over about 333 nanometers according to the Australian Museum, diffraction only happens in the infrared, which our eyes cannot see.

What is the difference between common opal and precious opal?

Common opal has the same basic chemistry but no play of color, because its spheres are uneven in size or randomly packed. Without order, the light waves never line up into pure colors. The Australian Museum notes that most of Australia's opal is this common opal, called potch, with precious opal only a small percentage. Australia supplies about 95% of the world's precious opal, with small amounts from Mexico, Brazil and the United States.

I find this the most beautiful part of the story, coming from physics and biology: the material is identical. The only difference between a plain stone and a flashing one is order at a scale a thousand times smaller than a hair.

How is opal's color different from a diamond's sparkle?

A diamond's fire comes from dispersion: its high refractive index bends each color of light by a slightly different amount, so white light fans out into a spectrum as it bounces between facets. Opal's color comes from diffraction by its inner structure, which is why even a smooth, rounded opal shows color without any facets at all. GIA lists opal's refractive index at 1.37 to 1.47, much lower than diamond's. We explain diamond's side of the story in Why Do Diamonds Sparkle? The Physics of Light in Your Jewelry.

How do you care for opal jewelry?

Because opal contains water, the Australian Museum warns that it may crack if exposed to sudden or extreme changes in temperature or humidity. GIA lists its hardness at 5 to 6.5 on the Mohs scale, far softer than diamond at 10. In practice:

• Avoid leaving opal in a hot car or near a heater, a real risk in a Los Angeles summer.
• Store it apart from diamonds and harder gems that can scratch it.
• Clean it gently with a soft, damp cloth, not ultrasonic cleaners.

Waves you can wear

Play of color is, at heart, a story about light waves meeting and adding up. That is the same idea behind our Wavy collection, which began as the sine and cosine curves I studied in physics and became gold. We work in gold and natural diamonds rather than opal, but the physics of waves runs through every Wavy piece.

The Wavy Bow Diamond Hoops tie a wave into a bow, with approximately 1.85 ct of ethically sourced natural diamonds in 18K recycled gold. For something lighter, the Wavy Diamond Hoop Earrings curve a line of diamonds around the ear, motion made permanent in 18K recycled gold.

Margaret Krinetz Fine Jewelry, handmade in Tel Aviv and shipped to Los Angeles, makes every piece to order. If light and waves speak to you, explore the collection.

Frequently asked questions

Is opal the October birthstone?
Yes. GIA lists opal as an October birthstone.

Why does my opal look different from every angle?
Its grid of silica spheres reinforces a different wavelength of light at each viewing angle, so the color shifts as you move.

How long does delivery of an MKFJ piece to Los Angeles take?
Every piece is handmade to order in 4 to 6 weeks, then shipped by DHL Express in 3 to 5 business days, free on orders over $1,500.