How Is Opal Formed?

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Opal is one of the world’s most enchanting gemstones, famous for its kaleidoscope of color flashes that seem to dance across the stone. For centuries, people have asked the same question: how is opal formed? The answer lies deep within geology, chemistry, and time.

This guide will explain the opal formation process, focusing on the difference between Australian opals, formed millions of years ago in the Great Artesian Basin, and Ethiopian hydrophane opals, which formed through volcanic processes. By the end, you’ll understand not only what opal is made of but also where opals come from, how long it takes for opals to form, and why Australia produces the world’s finest specimens.

What Is Opal Made Of?

At its core, opal is not a mineral but a mineraloid. Unlike crystalline gemstones such as sapphire or diamond, opal lacks a defined crystal structure. Instead, it is composed of tiny spheres of silica (SiO₂·nH₂O) stacked together, with water content ranging from 3% to 20%.

These microscopic silica spheres diffract light, creating the famous phenomenon known as play of color; the rainbow flashes that make opal so unique. The size and arrangement of these spheres determine the color:

  • Smaller spheres produce blues and violets.
  • Medium sized spheres display greens and yellows.
  • Larger spheres create reds, the rarest and most valuable hue.

So when someone asks, “what makes opal?” the answer is: water, silica, and time.

The Opal Formation Process in Nature

How does opal form in nature? The process begins when water seeps into the earth, dissolving silica from sandstone and other rocks. This silica rich solution travels through cracks, voids, and cavities in the ground. Over thousands or even millions of years, the water evaporates, leaving behind a deposit of silica.

If the silica particles arrange in an orderly, grid like structure, they diffract light and form precious opal. If the structure is random, the result is common opal (also called potch), which lacks the dazzling color play.

This basic process has happened in many parts of the World, but the conditions that created Australia’s opals are very different from those in volcanic regions like Ethiopia.  

formation of opal

How Is Opal Formed in Australia?

Australia produces over 90% of the world’s opals, including the most valuable black opals from Lightning Ridge, boulder opals from Queensland, and white opals from South Australia. But what makes Australian opals so unique is their geological origin.

Around 100 million years ago, much of central Australia was covered by a vast inland sea known as the Great Artesian Basin. As the sea slowly evaporated, it left behind silica deposits. Over millions of years, groundwater carried these silica solutions through ancient sandstones, filling cracks, voids, and even the remains of fossils.

This process is why opalized fossils, dinosaur bones, marine shells, and even ancient plant material are sometimes discovered in Australian opal fields. The opal literally replaced the organic material, preserving it in stunning rainbow colors.

Key points about Australian opal formation:

  • Formed in sedimentary rocks within the Great Artesian Basin.
  • Occurred during the Cretaceous period, around 100 million years ago.
  • Silica rich groundwater deposited opal in cracks, ironstone host rock, and fossils.
  • This slow natural process created the stable, non porous structure of Australian precious opal.

This explains why black opal from Lightning Ridge and boulder opal from Queensland are so durable and highly prized; they formed under ideal geological conditions over immense spans of time.

Great Artesian Basin
Image of the Great Artesian Basin Courtesy of CSIRO

How Are Ethiopian Hydrophane Opals Formed?

In contrast, Ethiopian opals, first discovered in the early 1990s and later made famous by the 2008 Wello discovery, formed under volcanic conditions rather than sedimentary.

Instead of an ancient inland sea, Ethiopian opals formed when silica rich fluids interacted with volcanic ash and rhyolite. Chemical reactions within the volcanic rock created cavities and nodules that filled with opal.

Most Ethiopian opals are hydrophane opals, meaning they can absorb water like a sponge. When placed in water, they may temporarily become transparent or even change color. While this property fascinates collectors, it also makes Ethiopian opals more fragile, sensitive to cracking, and less stable compared to Australian stones.

Key points about Ethiopian opal formation:

  • Formed in volcanic rock, not sedimentary sandstone.
  • Much younger than Australian opals (likely within the last few thousand years).
  • Hydrophane structure allows high water absorption (up to 15%).
  • Chemically reactive and less stable than Australian opals.

This difference in formation explains why Australian black opals are considered the most valuable and investment worthy, while Ethiopian hydrophane opals are more affordable but require extra care.

how is opal formed

How Long Does It Take for Opal to Form?

The timeline for opal formation depends on its environment. In Australia, opals formed slowly over tens of millions of years as groundwater cycled through ancient sandstones. Ethiopian opals, by contrast, formed much more rapidly in volcanic rocks, potentially within thousands to hundreds of thousands of years.

This geological timescale is part of what makes each opal unique; a gemstone shaped by natural forces across vast stretches of time.

Opalized Fossils: Nature’s Rarest Creations

One of the most remarkable results of the opalization process is fossil preservation. When silica replaced the organic material of ancient plants and animals, it created opalized fossils. Examples include opalized shells, wood and vegetation, and even full dinosaur bones.

These specimens are found almost exclusively in Australia, further highlighting how the Great Artesian Basin created conditions found nowhere else on earth.

Where Are Opals Found in Australia and the World?

  • Australia – Lightning Ridge (black opal), Coober Pedy (white opal), Andamooka, Mintabie, Queensland (boulder opal).
  • Ethiopia – Wello Province (hydrophane opal).
  • Mexico – Fire opal, formed in volcanic rocks.
  • Brazil – Precious opal deposits.
  • United States – Nevada and Idaho produce some opals, including opalized wood.

Still, Australia remains the global leader in opal production, both in quality and quantity.

Why Understanding Opal Formation Matters

Knowing how opal is formed helps buyers, collectors, and jewelers understand the differences between opal types, their stability, and their value. It also highlights why Australian opals, formed in an ancient inland sea over millions of years, remain unmatched in beauty and durability compared to volcanic hydrophane opals.

Whether you’re drawn to the rare fire red flashes of Lightning Ridge black opal or the glowing neon hues of Ethiopian hydrophane opal, each stone tells a geological story millions of years in the making.

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