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The Beauty and Mystery of Opal: Unveiling Its Formation and Fossilization

The Beauty and Mystery of Opal: How It Forms and Replaces Fossils

Opal is a remarkable mineraloid that has puzzled gemologists, geologists, and mineral enthusiasts alike for centuries. With its distinctive play-of-color and mesmerizing spheres, opal is a mineraloid that has captivated people’s attention since ancient times.

But what is opal, and how does it form? This article provides a comprehensive overview of opal, including its composition, structure, formation, and replacement in fossils.

Composition and Structure of Opal

Opal is a mineraloid, which means that it does not have a crystalline structure like typical minerals such as quartz or feldspar. Instead, opal is composed mainly of silica, the same compound that makes up quartz, but in an amorphous or non-crystalline form.

This lack of a defined crystal structure is what gives opal its unique properties, including its play-of-color, which is caused by the diffraction of light by tiny spheres of amorphous silica. Opal can be divided into two main types: precious opal and common opal.

Precious opal refers to opal that exhibits play-of-color, while common opal does not. Precious opal is prized for its beauty and rarity, while common opal is more abundant and used mainly for industrial purposes.

Opal’s structure is formed by millions of tiny spheres of amorphous silica arranged in a random or closed-packed pattern. These spheres can range in size from a few hundred nanometers to several micrometers in diameter and are responsible for the diffraction of light that produces opal’s play-of-color.

The color of opal depends on the size and arrangement of these spheres, with larger spheres producing colors in the red and orange range and smaller spheres producing blue and green colors. Opal also contains water, usually between 2% to 10% by weight, which is important for its formation and properties.

This water is chemically and physically bound within the silica structure and is responsible for opal’s characteristic iridescence.

Formation of Opal

Opal forms through a process called water precipitation, which occurs when water carrying dissolved silica (SiO2) passes through rocks and soil and collects in voids or spaces. As the water evaporates or flows away, the silica becomes supersaturated, which means that the concentration of silica in the water exceeds its solubility limit.

At this point, the silica precipitates out of the water and forms a gel, which fills the voids and solidifies over time. The gel transforms into opal due to a specific cooling process.

If the temperature decreases slowly, the gel fractures and contracts, forming regular arrays of amorphous silica spheres that diffract light and form the play-of-color. The amount of water and the cooling rate are critical factors that determine the type and quality of opal formed.

Subtle variations in temperature, pH, and mineral content can produce different types of opal, ranging from common opal to precious opal and different colors and patterns in the play-of-color.

Opal Replacement in Fossils

Opal has a unique property where it can replace organic materials such as wood, shells, and bones, preserving their structure and morphology. The process of opal replacement is called opalization and occurs when the organic material is gradually replaced by silica gel.

The process of opalization begins with the formation of a void or empty space in the organic material, such as a hollow shell or a decaying bone. The void provides a template for the formation of the silica gel, which fills the void and eventually solidifies into opal.

The opalization process may take thousands or millions of years, depending on the conditions and the size and shape of the void. Opalization can preserve the intricate details of the organic material, making it an essential tool for paleontologists and geologists to study the fossil record.

Petrified wood, for example, is a type of fossil that has been replaced by opal, forming an eerie replica of the original wood with the same texture, color, and cellular structure. Opalized shells and bones also provide valuable insights into the morphology and functions of ancient organisms, shedding light on their behavior, ecology, and evolution.


In conclusion, opal is a fascinating mineraloid with a complex structure and formation process. It owes its beauty and mystique to the interplay between silica spheres, water, and light that produce its mesmerizing play-of-color.

Opal also has the unique property of replacing organic materials, preserving their morphology and structure for millions of years. Understanding opal and its properties can reveal much about the Earth’s history and the evolution of life on our planet.

Whether we appreciate it for its intrinsic beauty or its scientific value, opal will continue to fascinate and inspire us for generations to come.

3) Opalized Revil from Personal Collection

Opalized ammonites are some of the most sought after and impressive fossil specimens for their striking iridescence and preservation of the ammonite’s nacre layer. Ammonites are extinct marine animals that lived millions of years ago during the Jurassic and Cretaceous periods.

Their shells were composed of aragonite, a type of calcium carbonate, and were intricately patterned with ridges and frills. When these shells are replaced by opal, they retain their original structure but are transformed into a shimmering display of opal play-of-color.

Opalized ammonites are found in various locations worldwide, with some of the most impressive specimens coming from Australia. One such specimen is the opalized pliosaur, a type of marine reptile that lived during the Cretaceous period.

It is one of the most significant opalized fossils ever discovered, displaying a nearly complete skeleton that has been replaced by opal. The specimen is estimated to be around 100 million years old and is valued at over $1 million.

Another example of an impressive opalized fossil from Australia is an opalized ammonite called the “Rainbow of Death.” This specimen was discovered in the Andamooka Opal Fields in South Australia and is known for its vivid display of opal play-of-color. The Rainbow of Death is an ammonite species named Craspedites glaucus, and its opalized shell is estimated to be around 120 million years old.

Opalized fossils are not limited to Australia, however. The Royal Tyrrell Museum in Alberta, Canada, is home to several impressive opalized specimens, including an opalized dinosaur tooth and an opalized turtle.

These fossils were preserved by the unique conditions of the region, which allowed for the replacement of the original bone and shell material with silica and opal.

4) Opalized Creature Discoveries

Opalized creatures, including ammonites, pliosaurs, and even new species, have been discovered in various locations worldwide. Australia is one of the most fruitful areas for opalized fossils due to its rich opal deposits and ancient geological history.

One of the most significant opalized fossils ever discovered is the opalized pliosaur mentioned earlier. This nearly complete skeleton was discovered in the opal fields of Lightning Ridge in New South Wales, Australia, by miner Bob Foster in 1987.

The opalized pliosaur is estimated to be around 100 million years old and is the only known specimen of its kind in the world. It is also one of the largest opalized fossils ever discovered, measuring nearly 5 meters in length.

In 2020, a new species of opalized marine reptile was discovered in Queensland, Australia. The specimen, named Thalassodraco etchesi, is thought to be an early member of the Pliosaur family and is estimated to be around 115 million years old.

The discovery of this new species highlights the importance of opalized fossils in understanding the evolution and diversity of marine reptiles during the Cretaceous period. Opalized creatures have also been discovered in other parts of the world, such as Madagascar, where an opalized turtle was found in the southern region of the island.

This specimen is thought to be around 80 million years old and was preserved by the unique geological conditions of the region. In conclusion, opalized fossils are not only impressive and beautiful specimens but also important tools for understanding the Earth’s geological and biological history.

They offer a unique glimpse into the diversity of ancient life, as well as the environment and conditions in which they lived and died. With recent discoveries of new species and impressive specimens, there is still much to learn and discover about opalized creatures and their significance in the fossil record.

In conclusion, opal is a mineraloid that has captivated people’s attention for centuries due to its unique play-of-color and mysterious formation process. Opalized fossils, including ammonites, pliosaurs, and new species, provide valuable insights into the Earth’s geological and biological history and shed light on the diversity and evolution of ancient life forms.

With their beauty and scientific significance, opalized fossils are a testament to the wonders and mysteries of our planet’s past. FAQs:

Q: What is the difference between precious and common opal?

A: Precious opal exhibits a play-of-color, while common opal does not. Q: What is opalization?

A: Opalization is the process of replacing organic materials with silica gel, resulting in opalized fossils. Q: Where have opalized fossils been found?

A: Opalized fossils have been found in various locations worldwide, including Australia, Alberta, and Madagascar. Q: What are some impressive opalized fossils?

A: Some impressive opalized fossils include the opalized pliosaur skeleton, Rainbow of Death ammonite, and opalized dinosaur tooth. Q: Why are opalized fossils significant?

A: Opalized fossils offer valuable insights into ancient life, the environment, and conditions in which they lived and died, and the Earth’s geological and biological history.

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