What is the white coating "patina" we often see on deposited chalcedony/agate? 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:





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.
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:


https://en.wikipedia.org/wiki/Opal
https://rruff-2.geo.arizona.edu/uploads/CM46_139.pdf
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.