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Chatoyance in obsidian?

Hi folks, I recently acquired some assorted slabs for cabbing.
Among them is some black obsidian with mahogany swirls. The
patterning reminds me of pietersite or Australian tiger iron.
There are areas of chatoyance similar to tiger eye (asbestos in
glass?) and occasional dots(2mm) of color like that in fire
agate. It makes beautiful cabs. Does anyone know where this
material comes from or how it was formed? Regards, Larry

It sounds like the stuff that comes from right here in the
Nevada desert. Don’t know much about it except that it is only
found in Nevada. Our local Reno Gem and Mineral Society can
provide you with more info and posibly be able to sell you some.
The society members dig it up themselves. Reno Gem and Mineral

480 S. Rock Blvd.
Sparks, NV. 89431
(702) 356-9864

God Bless, Donna

Obsidian is technically a volcanic glass, so therefore ideally
has no crystals. Over time, or with different cooling rates,
crystallization (devitrification) begins within the almost-solid
or completely solid glass. Depending on how well-mixed the
original melt is, devitrification can proceed along streaks or
bands in the glass, or in expanding snowflake-like patterns. I’ve
looked at some snowflake material under high magnification
(1000X, transmitted light), and the “flake” areas are full of
little embryonic crystals called trachytes(? sp) - they look like
little claws - really odd. But regarding the chatoyance and the
"rainbow" effects: Both are some sort of diffraction effect due
to some microstructure within the material. My guess would be
that chatoyance is due to the inclusion of a whole lot of little
parallel crystal needles - that’s why it’s so directional. And
the rainbow effect may be due to the inclusion of millions of
very tiny bubbles, laid out in parallel layers so that they have
the effect of a diffraction grating.

BTW - really glassy obsidian, with no crystals or bubbles, is in
fact translucent where fairly thin. It’s about the color of
cloudy smoky quartz.