How Do Opalised Fossils Form

Opal forms in cavities within rocks. If a cavity has formed because a bone, shell or pinecone was buried in the sand or clay that later became the rock, and conditions are right for opal formation, then the opal forms a fossil replica of the original object that was buried. 

Opalized fossils are formed through a process called opalization, which involves the replacement of organic material with silica, the main component of opal. This process typically occurs in two main ways:

Internal details not preserved: 

Internal Molding: When an organism, such as a shell or bone, is buried in sediment, the organic material can decompose, leaving an empty cavity. Over time, silica-rich water seeps into the cavity, depositing tiny spheres of silica that gradually fill the space. As these spheres pack together tightly, they form a solid opal replica of the original organism. This type of opalized fossil is known as an "internal mold" because it preserves the external shape of the organism but not its internal structure.

Internal details preserved: 

Replacement of Organic Material: In some cases, silica-rich water can replace the organic material of an organism without dissolving it entirely. This process is known as petrification or replacement. The silica molecules gradually infiltrate the organic structure, replacing the original atoms and preserving the fine details of the organism's internal and external features. This type of opalized fossil is known as a "replacement opal" because it faithfully replicates the original organism in its entirety.

How Do Opalised Fossils Form
This is a crab claw that has been opalised in the same way that petrified wood is. This is a very rare and awesome find. Photo:  Peter Cuneo

Factors that can influence the formation of opalised fossils:

  • The presence of silica-rich fluids: The availability of silica-rich water is essential for opalization to occur. These fluids can come from various sources, such as hot springs, geysers, or weathering of silica-rich rocks.

  • Permeability of the surrounding rocks: The surrounding rocks need to be porous enough to allow silica-rich fluids to flow through them and reach the organic material.

  • Stability of the environment: The environment in which opalization occurs needs to be relatively stable to allow the process to proceed slowly and without disruption.

Opalized fossils are relatively rare and are often found in areas where there has been significant volcanic activity, as hot springs and geysers can provide the silica-rich fluids needed for opalization. These fossils are valued for their beauty and scientific significance, providing a glimpse into ancient ecosystems and preserving delicate organic structures that would otherwise decay over time.

Opalised Fossils

Opalised Fossils

Opalized shells
Opalized freshwater mussel shells, Lightning Ridge, Australia

 Opals are highly sought-after rainbow precious stones that are showcased in jewellery or decorative art.

See also:
Why is Australian opal Unique?
Why Fluorite Comes in Different Colors? With Examples
Types of Mineral Inclusions with Photos
How Do Asterism Minerals Form?
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