Opalized Petrified Wood

Colorful crystal patterns are displayed in a cross-section of petrified wood in the Petrified Forest National Park, located in northeast Arizona. Photo by: Patrick Fuchs

Wood opal is a form of petrified wood which has developed an opalescent sheen or, more rarely, where the wood has been completely replaced by opal.

How is such a transformation possible? Well, it can only take place under the right set of circumstances.

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.

See also: 
How Did Those Petrified Logs Get 'Cut'?
What Is Petoskey Stone, and Where Can You Find It?
Ringing Rocks A Geological and Musical Marvel