What’s Inside is What Counts !
Lots of Rocks have a coating that makes it difficult to see what's inside them. Thundereggs look like lumps of mud. There is a “ThunderEgg Days” festival held in the small town of Nyssa, Oregon where you can buy a thunderegg and have it cut open right there just to see what is inside. The Berber agate (If none are currently listed for sale, send me a message.) has a thick, powdery white coating. These are not, strictly speaking, “coatings”. They’re part of the rock.
One of rockhound skill is to have the understanding and experience to imaging what a rock is really like – inside. I have a thousand page book about thundereggs. The author invests quite a few pages to describing how to orient a standard thunderegg for cutting to get the best out of it.
When a rockhound refers to “rock coating” or “patina”, it’s usually “desert varnish”. A lot of rocks that have laid out in the sun for a few thousand years have a thick coating. My dad’s “Pigeon Blood Agate” is a good example.
Take this piece of Pigeon Blood agate. The exposed surface looks mainly dull black. That's desert varnish.
Depending on conditions and rock, it can take thousands of years to form. Science still doesn’t completely understand how it happens.
Desert varnish is a mineral coating that contains iron and manganese oxides. The amount of manganese is 50 to 60 times more than what occurs naturally in the environment, so something must happen to concentrate it in the coating. Early investigators thought that the varnish came out of the rocks itself. Today, many sources say that bacteria are responsible. For example, the National Park Service page clearly states, “Bacteria take manganese out of the environment, oxidize it, and cement it onto rock surfaces.” NPS also states that the main component of desert varnish is clay.
Wikipedia has a completely opposite conclusion.
It has been suggested that desert varnish should be investigated as a potential candidate for a "shadow biosphere". However, a 2008 microscopy study posited that desert varnish has already been reproduced with chemistry not involving life in the lab, and that the main component is actually silica and not clay as previously thought.
Later, in the same Wikipedia article I found this statement:
Microscopic and microchemical observations, however, show that a major part of varnish is clay, which could only arrive by wind.
… and …
One proposal for a mechanism of desert varnish formation is that it is caused by manganese-oxidizing microbes (mixotrophs) which are common in environments poor in organic nutrients.
Wikipedia should make up its collective mind about this.
To resolve the contradiction in my own mind, I invested some time following the links and reading about it. I found the term “autotrophic” in some of the more scientific articles.
An autotroph is an organism that can produce its own food from simple inorganic substances such as carbon dioxide and water, without relying on other organisms for nutrition.
They have discovered autotrophs inside the Chernobyl atomic reactors that eat radiation. They have discovered them in deep sea hydrothermal vents that eat chemicals from volcanic magma. It seems likely then that existing on the desert shouldn’t be too much of a trick. I’m going with the National Park Service.
A freshly broken piece of Pigeon Blood looks like this instead. Quite a difference, don't you agree!
Montana Agate – A varnish of a different color
Montana agate has an entirely different coating and it's a result of a completely different environment – the Yellowstone River! The coating on Montana agate can be much thicker than desert varnish on rocks like Pigeon Blood agate and it's white instead of black. Nearly all Montana has it; it's one way that the rock can be identified.
Consider this piece. The top side has a typical, thick coat. The bottom side has virtually none. So, being exposed must have something to do with it. I checked a piece with sulfuric acid that will eat right through rocks like limestone or calcite. It had no effect.
Montana agate has spent thousands of years rolling down the Yellowstone River, not sitting in the hot, dry desert sun. But both need to be exposed before the coating will form.
I found a forum discussion where rockhounds were wondering why a very similar coating would form on native American arrowheads made of agate that were found in north west Washington state. They also noticed that it only formed on the exposed side and, since it was found on an artifact, it had to be only a few thousand years old. Whatever process creates this coating must be relatively fast in geological terms.
I also have chunks of a fern fossils that date from around 150 million years ago. There is an orange coating on them!!! And, no, it’s not lichen. It’s about like desert varnish except that it’s orange. Like both Pigeon Blood Agate and Montana Agate, it seems like the coating only forms on the exposed surface.
As nearly as I can tell, a lot of research has been done on this stuff with very little reliable results – with the possible exception of scientific papers and academic degrees for the contending scientists.
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