Opal: The Gemstone of Moving Fire

October’s birthstone is often described simply as colorful or delicate.

But opal is unlike any other gemstone used in jewelry.

Instead of forming as a solid crystal like sapphire or emerald, opal forms from silica-rich water slowly solidifying inside rock. The result is a gemstone capable of displaying one of the most extraordinary optical effects in nature: play-of-color — flashes of shifting rainbow light that move as the stone is viewed from different angles.

No two opals ever display the exact same pattern.

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Born From Water

Most gemstones form deep underground under intense heat and pressure.

Opal forms in a completely different way.

When water rich in dissolved silica seeps into cracks and cavities in rock, the silica gradually deposits layer by layer. Over time, this material solidifies into opal.

Unlike crystalline gemstones, opal is technically a mineraloid, meaning it lacks a regular crystal structure.

Key Gemological Properties

Composition: Hydrated silica (SiO₂·nH₂O)
Water content: typically 3–21%
Mohs hardness: 5.5–6.5
Structure: Amorphous (non-crystalline)

The water content contributes to opal’s unique appearance but also explains why it requires gentler care than many other gemstones.

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The Science of Play-of-Color

The flashes of color seen in precious opal are not caused by pigments or trace elements.

Instead, they result from microscopic spheres of silica arranged in a regular pattern inside the stone.

When light enters the gem, it bends and diffracts through these tiny spheres, separating into different wavelengths of color.

This optical effect produces the shifting rainbow flashes known as play-of-color.

The size and arrangement of the silica spheres determine which colors appear and how strong the effect is.

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Opal in Ancient Myth and Legend

Because of its shifting colors, opal has inspired myths across many cultures.

Ancient Romans believed opal contained the beauty of all gemstones combined. In medieval Europe, it was considered a stone of good fortune and prophecy.

Later in the 19th century, a novel titled Anne of Geierstein portrayed an opal as a cursed gemstone, which briefly damaged its reputation in European markets.

Despite this superstition, opals continued to be prized by collectors and royalty. Queen Victoria was known to gift opal jewelry to members of her family.

Today, the idea of opal being cursed is widely recognized as a historical myth rather than a real belief within the gem trade.

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Opalized Fossils: Nature’s Time Capsules

Australia, the world’s leading source of precious opal, is also famous for an unusual phenomenon: opalized fossils.

In some cases, ancient fossils become replaced by silica-rich solutions that eventually solidify into opal.

Shells, wood, and even dinosaur bones have been discovered preserved in opal, creating spectacular specimens where prehistoric structures display vibrant play-of-color.

These rare formations combine paleontology and gemology in a single object.

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Famous Opal Sources

The majority of the world’s finest opals come from Australia, particularly:

• Lightning Ridge – famous for black opal

• Coober Pedy – known for white opal

• Andamooka – produces crystal opal

Other sources include:

• Ethiopia

• Mexico

• Brazil

• Nevada (United States)

Among these, Lightning Ridge black opal is often considered the most valuable due to its dark body tone and intense color contrast.

Fine examples can reach thousands of dollars per carat.

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Why Opal Requires Special Care

Compared to many gemstones, opal is somewhat delicate.

With a hardness of 5.5–6.5 on the Mohs scale, it can scratch more easily than sapphire or diamond.

Opal is also sensitive to:

• sudden temperature changes

• prolonged dryness

• impacts or pressure

For this reason, opal jewelry is best stored carefully and cleaned gently with mild soap and water rather than ultrasonic cleaners.

With proper care, opal jewelry can remain beautiful for generations.

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Why Opal Remains One of the Most Unique Gemstones

Few gemstones display as much individuality as opal.

Each stone contains a unique pattern of color, flashes, and movement that cannot be duplicated. This unpredictability is part of what makes opal so captivating to collectors and jewelry lovers alike.

From ancient myths to modern gemstone science, opal continues to stand apart as one of the most distinctive gems ever discovered.

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Frequently Asked Questions About Opal

What causes opal’s play-of-color?

Play-of-color occurs when light diffracts through microscopic silica spheres arranged inside the gemstone.

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Are opals fragile?

Opal is softer than many gemstones and can be sensitive to impacts or extreme temperature changes. Proper care helps maintain its durability.

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What is black opal?

Black opal refers to opal with a dark body tone, which enhances the visibility of play-of-color. The finest examples come from Lightning Ridge in Australia.

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Do opals really contain water?

Yes. Opal can contain several percent water within its structure, which contributes to its classification as hydrated silica.

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Are opals rare?

Fine opals with strong play-of-color, especially black opals, can be quite rare and valuable.