Subject: Re: Diamond, or not Diamond?

From: okunewck@pds16 (Phil OKunewick)
Date: 2 Mar 94 16:46:03 GMT

[email-masked] (Bill Blohm) writes:
>Phil OKunewick (okunewck@pds16) wrote:
>
>> >1) What is this stone?  (Scientific name so I can look up more)
>
>> Can't say from the information given.  It could be obsidian, aka
>> "Volcanic Glass" - this is black and glassy, and is used in low-cost
>> jewelry.  It could be a number of other things too.  The value and the
>> rating depend on what it actually is, though I would guess offhand
>> that a stone which a kid is likely to pick up wouldn't be worth a
>> whole lot.  Still, there are kids who get lucky.
>
>No, I don't think it is obsidian.  It was found as an oval, and there
>were several others around.  The area had what is called black sand
>and was selected by my parents as they were hunting gold for the 
>weekend.  I have a big chunk of obsidian, and this is nothing like
>it.  Obsidian that I have seen seems to be more flat planed while 
>this is more like a lobe.  It just happened to be the biggest one,
>and that is why I kept it.  There were lots of them in the sand and
>dirt, but they were all like misshapen balls, rarely getting over 1/4"
>if I recall.

   Rocks that are tumbled by water tend to become rounded like you
describe here.  Water tumbling almost completely obliterates any
normal fractures as it makes everything round.  Glacial tubmling
does the same thing.  (hard water? :-)

   Obsidian is not a very specific mineral; it's essentially fused
quartz and feldspar which rapidly cooled.  It's called Volcanic Glass
because it looks a lot like randomly poured black glass (similar to
taffy).  And as such, it can take on all sorts of shapes and fracture
patterns.  The "proper" fracture is conchoidal (like a BB-hole in a
window) but this can be affected to what happened as it formed.  An
interesting deposit of this is Obsidian Cliffs in Yellowstone, where
the obsidian bedrock is like columnar basalt formed into foot-wide 4-6
sided vertical columns, and the loose obsidian is in all sorts of
random broken shapes.

   On the other hand, there is a "black sand" assiciated with gold,
which is almost as heavy as gold.  Gold is usually found in igneous
intrusive rocks (granite), while obsidian is extrusive (lava).  But
when minerals wash out of rock, you get a fine mess of every rock
formation for hundreds of miles upstream.  I can't remember the name
of the black sand, but I believe it contained magnesium or manganese;
not sure.  Specific gravity of gold is 19.3; you can test your rock
with a postage scale, a string, and a glass of water (check a mineral
book for more details).  If the s.g. is over 18, it's probably this
"black sand".

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