[Rockhounds] Yellow & green obsidian

[Charles Baran]

So, what is the inclusion that causes mahogany obsidian?  I have 
gathered some from Glass Butte Oregon that is about 90% mahogony and 10% 
black.  I have also gathered some that is 33% each of gold 
sheen/mahogany/rainbow.  All from Glass Butte.  Chuck

Rock Currier wrote:

>I have never seen any obsidian other than black or a smoky color like you
>commonly encounter in "apache tears". The other colors of obsidian are to
>the best of my knowledge caused by inclusions in the obsidian that give rise
>to varieties like the "snow flake" from Utah, the "mahogany" reddish-brown
>obsidian or the various of the "rainbow" colors of sheen obsidians. These
>last can be many colors and quite striking reds, greens, violet, purple or
>the silver or golden sheen obsidians from Mexico. All of these obsidians are
>basically black and the colors are seen best when the obsidian has been cut
>more or less parallel or at a shallow angle down through the "color bands"
>in the obsidian. Much glass, especially the green glass produced in
>abundance in Mexico and other places is a pretty transparent green color and
>is commonly sold as green obsidian. I don't think there is any transparent
>natural green obsidian.
>
>At the GIA we were taught that the way you can definitely separate man made
>glass from natural materials is to look for completely round gas bubbles
>which they contend, or at least used to contend was the hall mark of man
>made glasses. I think this was based on the idea that flow of natural
>glasses in their molten state would stretch any included bubbles out of
>round. This is a rule of thumb that I have sometimes used to separate glass
>from natural materials. I am not sure that some natural glasses could not
>have round bubbles, but I don't think I have ever seen any. Certainly the
>vesicles in the obsidian from Little Lake, California that are commonly full
>of feldspar, cristobalite and fayalite are not completely round.
>
>Tektites are another category of natural glass, but they usually have
>distinctive surface features that set them apart from man made glasses. The
>transparent green of the moldavites from the Czech Republic are very
>distinctive when combined with their rough surface features. The black
>indochinites can be had so cheaply in bulk that I don't think it has ever
>occurred to anyone to try and go to the effort of trying to make them.
>
>There is a type of yellow tektite, sometimes transparent, from Libya,
>sometimes called Libyan desert glass that is authentic. Small pieces of this
>have been on the market for several years. Some of this material may have
>been marketed as yellow obsidian, but I would think that calling these
>yellow obsidian would cause them to sell for less on the market than calling
>them by their more commonly accepted name.
>
>Rock
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