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

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

These are no ordinary fossils (if there is such a thing): these incredible relics are made of solid opal, sometimes with rainbows of shimmering colour.

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. We get opalised fossils of two kinds:

1. Internal details not preserved: 
 Opal starts as a solution of silica in water. If the silica solution fills an empty space left by a shell, bone etc that has rotted away – like jelly poured into a mould – it may harden to form an opalised cast of the original object. Most opalised shell fossils are ‘jelly mould’ fossils – the outside shape is beautifully preserved, but the opal inside doesn’t record any of the creature’s internal structure.

2. Internal details preserved: 
If the buried organic material hasn’t rotted away and a silica solution soaks into it, when the silica hardens it may form an opal replica of the internal structure of the object. This happens sometimes with wood or bone.

How Do Opalised Fossils Form

How Do Opalised Fossils Form

How Do Opalised Fossils Form
Opalized freshwater mussel shells, Lightning Ridge, Australia
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?