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Opalized Fossils: How Do They Form (With Examples!)

opalized fossils

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All About Opalized Fossils

Fossils are an amazing look into the past. As rockhounds, many of us learn about the different types of mineral replacement and how they end up. Why a petrified piece of wood is petrified for instance. Among the most prized petrified relics of the past are those that end up opalized, replaced with amorphous silica in a special configuration.

So, let’s take a look at opalized fossils and how they form, and then I’ll show you an example from my personal collection!

Keeping Opal Defined

Opal has a fairly strict definition that differentiates it from similar stones. It’s comprised mainly of silica, just like agates, chalcedonies, and even quartz crystals. The difference is in how it all fits together.

Opal is technically a mineraloid as it lacks a crystalline structure, one of the defining features of a mineral. This puts it into the same category as stones like the volcanic glass known as obsidian. Obsidian itself is also a silica mineral, usually high in iron which gives it the tell-tale dark color.

Opal is comprised of microscopic spheres stacked together. In some cases, light passes between the spheres, bending infinitesimally, and creating the play-of-color that precious opal is known for. It takes a long time for this to occur, as the spheres of precious opal need to be a uniform size.

Most opal is not precious opal, even in areas where precious opal is found. Instead, common opal lacks any play of color and more closely resembles a softer jasper than what we normally think of as opal. 

Common opal is sometimes called potch, especially in areas where precious opal is mined regularly.

The silica in opal is also hydrated, which means there are water particles trapped in the interior of the stone. This is why some opals react to water, increasing in transparency or even losing their play of color depending on the individual stone. The water isn’t in pores in the stone but is instead trapped in the structure itself.

Related: What is a Jelly Opal?

The important thing to keep in mind is that opal is a mineraloid composed of microscopic spheres of silica bonded together. Most opalized fossils are made of common opal, with precious opal replacements being an extreme rarity.

Opal’s Structure and Formation

Opal forms in a different way than most other silica stones. Quartz and chalcedony both form under heat, requiring a slower cooling process. 

Quartz crystals require the most heat to form. Think of it something like water being frozen into ice over time, as it slowly cools and turns into a larger crystal. Chalcedony follows a similar pattern in many cases.

Opals seem to form under relatively cool conditions, or at least as cool as anything that deep underground can be. The mechanism is thought to be water precipitation.

The Earth’s crust is over 50% silica, which is why it’s such a common ingredient in minerals and stones. As water seeps into the ground and becomes saturated with silica, it slowly trickles into voids and other spaces left behind in the bedrock around it. It then evaporates over time, leaving behind just the spheres and their impurities which are what we call opal.

The water may actually become supersaturated, meaning that it contains more than it will hold as it cools to the ambient temperature. The solution is closer to a gel than liquid water as it seeps downward.

There have to be some voids or some kind of porosity in the stone below for opal to form. The water/gel will slowly trickle in and fill any holes left behind in the bedrock during it’s formation or afterward.

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How Long Does Opal Take to Form?

Well, current science has the measure at about one centimeter per 5 million years. It’s not super slow, on a geological time scale, but it’s certainly not something I’d hold my breath waiting for.

The line between opal and chalcedony can be very thin, but hardness testing will usually give you a good idea. Opal’s lower hardness means it should be scratched by hardened steel (ie: a file, a good knife) while chalcedony cannot. It also appears that older opal may, in fact, become chalcedony as it crystallizes on a microscopic level.

Read More: Ultimate Guide To The Moh’s Scale of Hardness

Opal Replacement in Fossils

Replacement is what leaves us with the majority of fossils. The idea is pretty simple: something gets buried over the course of time, rots away in the environment underground, and creates a void that’s filled in with other minerals.

Chalcedony and opal are two of the most common minerals seen in replacements. The majority of hard petrified wood, for example, is comprised of chalcedony. In some places, however, opal is found instead. This can even vary with fossils from the same region.

I’ve tested pieces of Arizona Rainbow Confier in the past and found them as soft as opal, while others were impervious to anything softer than a quartz crystal.

Among the most commonly seen are woods and shells. Shells are particularly common, and almost uniformly lack the soft parts of the mollusks they came from. This happens because flesh rots much more quickly than the shell material, but the shell material is actually a carbonate mineral which is water soluble and will break down over time.

Most mollusk shells are composed of biogenic aragonite and calcite, which the creatures produce to protect themselves. The shells will then “rot” over another geological period of time before the water seeping in from above fills the area.

Bones and other hard parts of animals also have the potential for opalization, but it’s rarer. Still, some really impressive examples have been found, including the discovery of a new species in Australia from opalized remains.

The simple way to put it is that organic material rots, leaving a shaped void that’s filled with silica gel that slowly turns into opal. These processes are so slow that they’re not really observable on a human timeline, but we can certainly enjoy the end result of millions of years of work by Mother Nature!

Example: An Opalized Ammonite

I’m not much of a fossil guy, but I’ve always had a “thing” for ammonites, ever since I received my first one a couple of decades ago. This one is the prize of my (small-ish) collection, an opalized ammonite.

As you can see in the photo above it displays some iridescence. This is the same iridescence found in many ammonites found in Alberta and Madagascar and not an opal play-of-color. The white shell is actually common opal.

Oddly enough, it appears that some ammonoid fossils are replaced while preserving some of the nacre layer contained on the inside of the shells. You’ll sometimes see all iridescent ammonites described as “opalized” but this isn’t the case. Instead, it appears the aragonite’s structure has been preserved over the course of the opal’s formation.

Of course, this is far from the most dramatic example of an opalized creature. That honor falls to the Australian continent, where they’ve found such impressive skeletons as a fully opalized pliosaur skeleton!

Jeremy Hall
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