How Do Opalised Fossils Form?
![]() |
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:
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 areĆ ‘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.
![]() |
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?