Mineralization of Obsidian Artifacts

[bushape]

I recently sold an Obsidian Knife to a guy, this knife had a great deal of mineralization on one side of it. He called me and basically of misrepresenting the knife. He kept telling me that it wasn't obsidian and that it had been painted, he wouldn't listen to my thoughts on it. I told him that if an artifact lays on the ground long enough in the right circumstances that it can at times develop a great deal of calsification and mineralization on just one side.
I have seen this several times and he kept saying that he has never seen this or even heard of this in over 25 years of collecting. I tried to be nice and kept my tone down. He told me that it is my mistake, I just calmly told him to send it back and I would refund all of his money.
My question is: Is there somewhere that I could find info on this in order to show him that it really does happen?
I had one of the editor's to Overstreet's book look at and he was the one that did the write-up for my listing. It is really too bad that the guy didn't want this knife, because it was an X-Carrol Howe artifact. I just don't want a bad feedback for something that is totally legitiment.
Thank you

 

[GobyON]

My cousin's wife came moved to my area on the Great Lakes from Oregon, and nearly everyone of her arrowheads looked just like that. She told me that it was from a forest fire. I know they are authentic, but I don't know if her reasoning was true. I have seen quite a few obsidian points that looked exactly like the one you are listing. painted though? lol.. People will be people.

[turkeytail]

Seen it many many times on chert pieces but not that familiar with obsidian. Love your avatar by the way!!! :laugh:

[painshill]

Hi Stan
Obsidian is pretty resistant to weathering, except in very wet and/or alkaline soil environments. When it does weather, it normally suffers hydration, like this example which shows the weathered rind and a freshly-broken interior surface.

The hydration happens because obsidian is a type of glass which is naturally low in water content. The appearance moves from shiny black (although obsidian occurs in a number of other colours too) to pale grey, but that normally takes hundreds of years. Ultimately, a piece of obsidian can weather to perlite - which is almost white. Here's a highly weathered piece:

What you're seeing is more likely the surface alteration of the obsidian itself into minerals in the clay family, rather than a build-up of external patination. Because of that, obsidian artefacts can be dated fairly reliably (hydration dating) by measurement of the rind. It’s a useful and inexpensive test.
The pictures are from the website below, which has more information – including a link to a summary of the dating technique and a link to information on perlite:

http://geology.about.com/od/more_igrocks/ig/obsidian/obsidianrind.-183.htm