<p>The white &quot;patina&quot; is most likely some form of opal as you suggest. I did some research and I'll explain how the agate becomes opal and why it's more common in the eroded rock.
Some background:</p>
<ul>
<li>agate and opal are both forms of silica with the chemical formula SiO<sub>2</sub></li>
<li>opal is a hydrated amorphous form of silica (1)</li>
</ul>
<p>You would find the opal more often on weathered and deposited chalcedony because &quot;the polymerization of silica into three-dimensional gel-like networks [opal] is favored in the pH range 3 to 10 where dissolved salts are present in solution.&quot;(2) The pH of stream/river water ranges from 6.5 to 8.5 and all natural water has salts of some kind. (3) Therefore, dissolved silica in stream/river water reacts with the agate and opal grows onto it.</p>
<p>To sum up: yes, the &quot;patina&quot; is most likely opal and it forms when stream/river water interacts with the agate.</p>
<p>sources:</p>
<ol>
<li><a href="https://en.wikipedia.org/wiki/Opal" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Opal</a></li>
<li><a href="https://rruff-2.geo.arizona.edu/uploads/CM46_139.pdf" rel="nofollow noreferrer">https://rruff-2.geo.arizona.edu/uploads/CM46_139.pdf</a></li>
<li><a href="https://www.grc.nasa.gov/WWW/k-12/fenlewis/Waterquality.html" rel="nofollow noreferrer">https://www.grc.nasa.gov/WWW/k-12/fenlewis/Waterquality.html</a></li>
</ol>
<p>Source 2 talks about various types of opal formation, but for simplicity's sake I only mentioned polymerization.</p>

CC BY-SA 3.0
2017-02-10T09:16:18.990Z
Thank you very much for your answer. I think that we are close, but not there yet.

I do not think that the patina did formed in the river, but after the deposition. There is one great point in tne source that I have found (and provided the link under my photos) :



"Lots of artifacts* are patinated on one side, the side that faced up. The upward-facing side patinates faster than the other side because water collects on that side, while under the ground.."



* the artifacts are made of silica too
Petr Hykš
CC BY-SA 3.0
2017-02-20T12:59:21.280Z
Although I found possible solution now, your answer is based on possible usual material. I just say that my answer is quite different from yours. It's mycelium.
Takahiro Waki
CC BY-SA 4.0
2017-02-09T20:47:03.043Z
2023-03-06T10:20:41.677Z
2023-03-06T10:20:41.677Z
Brittan Wogsland
<p>In Arizona, I find this coating on agates/chalcedony. But, I think it forms in a reaction caused by the sun, as it will be on the top of the rocks and not on the underside in contact with the soil.</p>

CC BY-SA 4.0
2022-03-06T06:42:20.607Z
2022-03-06T06:42:20.607Z
Chris Vincent
<p>It is chemical weathering. It is caused by reaction to sunlight or caustic pH in soil that starts breaking down the non silica impurities in the agate. Heat from a natural forest fire can also cause this. You will often see many fractures from heat altering.</p>

CC BY-SA 4.0
2022-11-24T18:56:10.960Z
2023-03-06T10:20:58.447Z
2023-03-06T10:20:58.447Z
Johnny D
<p>Me and some of my friends were wondering what exactly is the white coating we can see on chalcedony that has been deposited by a river during quartenary (and so the original material is damaged and has a lot of cracks in it). This is an example:</p>

<p><a href="https://i.sstatic.net/jz4jW.jpg" rel="noreferrer"><img src="https://i.sstatic.net/jz4jW.jpg" alt="enter image description here"></a>
<a href="https://i.sstatic.net/cZyYa.jpg" rel="noreferrer"><img src="https://i.sstatic.net/cZyYa.jpg" alt="enter image description here"></a>
<a href="https://i.sstatic.net/Z424J.jpg" rel="noreferrer"><img src="https://i.sstatic.net/Z424J.jpg" alt="enter image description here"></a>
<a href="https://i.sstatic.net/Nxhl1.jpg" rel="noreferrer"><img src="https://i.sstatic.net/Nxhl1.jpg" alt="enter image description here"></a>
What is the white coating ? Is it opal ? Cacholong ?</p>

<p>So far everything I found was this link:
<a href="http://www.treasurenet.com/forums/north-american-indian-artifacts/238703-agate-arrowhead-white-patina-question.html" rel="noreferrer">http://www.treasurenet.com/forums/north-american-indian-artifacts/238703-agate-arrowhead-white-patina-question.html</a></p>

<p>These people describe it as "patina" or "chalcedony weathering". I am wodering what is the process and how can we easily chemically or physically explain it ? </p>

<p>I think it must be some kind of weathering.. similar to oxidation. I believe that it is opalization. What do you think ? I saw this forming on many agates all over the world, but only at these, that were eroded from the mother rock, transported and deposited. It is especiially common on deposited chalcedony.</p>

<p>Thank you.</p>

CC BY-SA 3.0
2017-02-09T13:58:00.813Z
This is a great question I had never considered before.  Many agates in my personal collection also have white coatings.  A quick search on Google Scholar did not reveal any useful technical article.  If I was in grad school, I would pursue this research.
Earth Science Expatriate
CC BY-SA 3.0
2017-02-09T22:23:37.720Z
Not oxidation. Silica is already oxidised, there's nothing more to oxidise there.
Gimelist
CC BY-SA 4.0
2022-03-06T21:58:23.077Z
This doesn't look like a coating, but simple internal banding, viewed face-on. Not at all unusual and typically forms concurrently with growth of the agate itself. Color variations are the result of fluctuations in trace element concentration and type, incorporated during crystal matrix growth.
Knob Scratcher
<geology><geochemistry><mineralogy><crystallography>
What is the white coating "patina" we often see on deposited chalcedony/agate?
Petr Hykš
# What is the white coating "patina" we often see on deposited chalcedony/agate?

(Asked by: username_0 on 2017-02-03)

Me and some of my friends were wondering what exactly is the white coating we can see on chalcedony that has been deposited by a river during quartenary (and so the original material is damaged and has a lot of cracks in it). This is an example:

[![enter image description here](https://i.sstatic.net/jz4jW.jpg)](https://i.sstatic.net/jz4jW.jpg)
[![enter image description here](https://i.sstatic.net/cZyYa.jpg)](https://i.sstatic.net/cZyYa.jpg)
[![enter image description here](https://i.sstatic.net/Z424J.jpg)](https://i.sstatic.net/Z424J.jpg)
[![enter image description here](https://i.sstatic.net/Nxhl1.jpg)](https://i.sstatic.net/Nxhl1.jpg)
What is the white coating ? Is it opal ? Cacholong ?

So far everything I found was this link:
<http://www.treasurenet.com/forums/north-american-indian-artifacts/238703-agate-arrowhead-white-patina-question.html>

These people describe it as "patina" or "chalcedony weathering". I am wodering what is the process and how can we easily chemically or physically explain it ?

I think it must be some kind of weathering.. similar to oxidation. I believe that it is opalization. What do you think ? I saw this forming on many agates all over the world, but only at these, that were eroded from the mother rock, transported and deposited. It is especiially common on deposited chalcedony.

Thank you.

**Tags:** geology, geochemistry, mineralogy, crystallography

> **Comments:**
> **username_1**: This is a great question I had never considered before.  Many agates in my personal collection also have white coatings.  A quick search on Google Scholar did not reveal any useful technical article.  If I was in grad school, I would pursue this research.  
> **username_2**: Not oxidation. Silica is already oxidised, there's nothing more to oxidise there.  
> **username_3**: This doesn't look like a coating, but simple internal banding, viewed face-on. Not at all unusual and typically forms concurrently with growth of the agate itself. Color variations are the result of fluctuations in trace element concentration and type, incorporated during crystal matrix growth.  

---

## 3 answers

### Answer by username_4 on 2017-02-09. Score: 6

The white "patina" is most likely some form of opal as you suggest. I did some research and I'll explain how the agate becomes opal and why it's more common in the eroded rock.
Some background:

* agate and opal are both forms of silica with the chemical formula SiO2
* opal is a hydrated amorphous form of silica (1)

You would find the opal more often on weathered and deposited chalcedony because "the polymerization of silica into three-dimensional gel-like networks [opal] is favored in the pH range 3 to 10 where dissolved salts are present in solution."(2) The pH of stream/river water ranges from 6.5 to 8.5 and all natural water has salts of some kind. (3) Therefore, dissolved silica in stream/river water reacts with the agate and opal grows onto it.

To sum up: yes, the "patina" is most likely opal and it forms when stream/river water interacts with the agate.

sources:

1. <https://en.wikipedia.org/wiki/Opal>
2. <https://rruff-2.geo.arizona.edu/uploads/CM46_139.pdf>
3. <https://www.grc.nasa.gov/WWW/k-12/fenlewis/Waterquality.html>

Source 2 talks about various types of opal formation, but for simplicity's sake I only mentioned polymerization.

> **Comments:**
> **username_0**: Thank you very much for your answer. I think that we are close, but not there yet.  I do not think that the patina did formed in the river, but after the deposition. There is one great point in tne source that I have found (and provided the link under my photos) :    "Lots of artifacts* are patinated on one side, the side that faced up. The upward-facing side patinates faster than the other side because water collects on that side, while under the ground.."    * the artifacts are made of silica too  
> **username_5**: Although I found possible solution now, your answer is based on possible usual material. I just say that my answer is quite different from yours. It's mycelium.  

---

### Answer by username_6 on 2022-03-06. Score: 1

In Arizona, I find this coating on agates/chalcedony. But, I think it forms in a reaction caused by the sun, as it will be on the top of the rocks and not on the underside in contact with the soil.


---

### Answer by username_7 on 2022-11-24. Score: 1

It is chemical weathering. It is caused by reaction to sunlight or caustic pH in soil that starts breaking down the non silica impurities in the agate. Heat from a natural forest fire can also cause this. You will often see many fractures from heat altering.


---

