[Rockhounds] Yellow & green obsidian

[Rock Currier]

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