Opal has long been admired for its shimmering iridescence, but what truly sets it apart from other gemstones is a breathtaking optical effect known as play of colour. This captivating visual dance of rainbow hues isn’t just magic—it’s science. Here's a deep dive into what causes this effect and why it's so unique.
What is Play of Colour?
Play of colour is an optical phenomenon that refers to the spectacular way opals interact with light. It produces brilliant flashes of neon-like color that shift and change as the gemstone moves. This dazzling characteristic is what makes precious opals so prized in jewelry and collections.
The Science Behind the Beauty
At its core, opal is composed of silicon dioxide and water. Unlike crystalline minerals, opal has an amorphous (non-crystalline) structure. However, some opals form tiny silica spheres that arrange themselves into a regular, repeating pattern within the stone.
These silica spheres are incredibly small—measured in nanometers—and their size and spacing directly affect the colors we see.
How Play of Colour Happens
The secret lies in light diffraction. As light enters the opal, it travels through the gaps between the stacked silica spheres. This forces the light to bend and split into its component wavelengths—similar to how a prism works. The result is a burst of spectral colors.
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Smaller spheres = shorter wavelengths like blue and green
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Larger spheres = longer wavelengths like yellow, orange, and red
The more uniform and structured the sphere arrangement, the more intense and vibrant the play of colour becomes.
Not All Opals Display Play of Colour
Only precious opals exhibit play of colour due to this unique internal structure. Common opals, which lack the ordered silica sphere arrangement, may have attractive hues but do not display the rainbow-like shimmer.
Factors Affecting the Quality
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Brilliance: How bright and vivid the colours are
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Pattern: The arrangement of colours (e.g., pinfire, harlequin, flash)
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Color Range: The number of colours visible
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Directionality: How the colour changes with movement
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Transparency: Some opals are opaque, others translucent to transparent
Why Opals Are Truly One of a Kind
Unlike other gemstones that reflect light, opals diffract it. That means the effect isn’t due to surface shine but the internal microstructure. This gives opal its signature glowing, “alive” look—something no other gem quite replicates.
