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 enter image description here enter image description here enter image description here 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.

  • 2
    $\begingroup$ 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. $\endgroup$ Commented Feb 9, 2017 at 13:58
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    $\begingroup$ Not oxidation. Silica is already oxidised, there's nothing more to oxidise there. $\endgroup$
    – Gimelist
    Commented Feb 9, 2017 at 22:23
  • $\begingroup$ 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. $\endgroup$ Commented Mar 6, 2022 at 21:58

3 Answers 3


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.


  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.

  • $\begingroup$ 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 $\endgroup$
    – Petr Hykš
    Commented Feb 10, 2017 at 9:16
  • $\begingroup$ 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. $\endgroup$ Commented Feb 20, 2017 at 12:59

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.


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.


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