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Fracture question - newbie



 
 
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  #1  
Old July 2nd 05, 02:20 AM
Cumberpach
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Default Fracture question - newbie

When looking into a say, a cabochon-cut amythist and you see wall-like
divisions, is this fault called a fracture ?
Is there a site which shows pics of faults etc ? TIA

Pete


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  #2  
Old July 2nd 05, 05:36 AM
Carl 1 Lucky Texan
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I'm no expert but there can be structures/inclusions called veils,
curtains, thumbprints, tubes, breadcrumbs, etc.

Check www.gemsociety.org
www.yourgemologist.com
www.rockhounds.com
for possible books and online assistance.

Carl


Cumberpach wrote:
When looking into a say, a cabochon-cut amythist and you see wall-like
divisions, is this fault called a fracture ?
Is there a site which shows pics of faults etc ? TIA

Pete



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  #3  
Old July 2nd 05, 05:58 AM
Peter W.. Rowe,
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On Fri, 01 Jul 2005 18:20:18 -0700, in |õ "Cumberpach"
wrote:

When looking into a say, a cabochon-cut amythist and you see wall-like
divisions, is this fault called a fracture ?
Is there a site which shows pics of faults etc ? TIA

Pete


Cabochon or facetted or other cutting style, or rough uncut condition has no
bearing on the nomenclature, so this applies to all.

There are, as Carl points out, many types and variations on inclusions or flaws or
internal features of a crystaline gem material (or the non-crystaline ones too.)
But of those that will resemble a fracture, there are a few worth mentioning.

First, if it's an actual break, or crack, or fracturer (call it what you wish),
then there are two principal types worth distinguishing.

First is just that, a crack or fracture, or whatever you wish to call it. They
follow the directions determined by the stresses that created them, be they damage
or just during formation of the gem. There are several common terms for these.
"feather" is a common one, especially in diamonds, since they often appear as
white feather shaped things in the diamond. They may also simply be called
fractures, cracks, or the like.

The second is also a full crack or break of this sort, but it occurs in those
materials where there are one or more specific directions where the material is
especially likely or easy to break. These directions are called cleavage planes,
and this type of break is called a cleavage. Some examples of stones where this
is possible are diamond (four possible directions), Topaz (one such direction),
fluorite (also four directions), Calcite forms, and a number of others. Amethyst
does not have any cleavage planes. Cleavages generally differ from other types of
fractures in that they follow a flat plane, while most other fractures wander in
curved or splintery or other random directions instead of being dead flat planes.
Cleavages are generally more serious in a gem because they represent a break that
has already started, in a direction that is substantially easier to break, so a
cleavage presents generally a significantly higher risk of being extended further
into the gem.

With both ordinary fractures or cleavages, if they form while the crystal is still
forming, it can be possible for the crystal growth process to partially heal the
break. Often this leaves fracture or cleavage shaped planes or curves that now
consist not of a single fracture, but of multiple small inclusions, sometimes
bubbles or tiny residual fractures, or the like. These can resemble whisps of
gauze floating in the gem, and are often called veils. They are still a source of
weakness in the gem, but considerably less of a risk than a full contiguous
fracture or cleavage.

Then there are two other common phenomena which can fit your description. Many
gem crystals are formed under somewhat less than ideal situations for a uniform
crystal. Chemistry in the zone may change, as the stone froms, such that color
from one part of the gem can change or vary. Sometimes the different colors or
intensity of colored areas in the crystal can be quite sharp demarkation zones,
which can appear as an internal "surface" or plane with no break or fracture, but
clearly seperating one colored area from a differntly colored, or differently
intense area of the same color. This is called color zoning. A dramatic example
of this might be Ametrine, where yellow citrine and purple amethyst can exist in
the same crystal, often clearly seperated by a sharp demarcation between colors.
Tourmaline too, is famous for this, with the so called watermelon trourmaline
showing red and green, and sometimes clear zones all in the same gem.

And finally, as crystals form, sometimes the atomic orientation of the crystal
structure can appear to switch. Often this is the result of two conflicting
crystal orientations trying to form at the same time, or two crystals intergrowing
together (sorry, that's a poor explanation) Either way, what results is called a
twinned crystal, so that the single stone switches it's atomic structure or
symmetry directions, usually along a planer direction in the stone. Sometimes it
will do it repeatedly as the crystal grows, so the end result looks like a stack
of layers. Because the optics of a gem can vary according to the direction
through the crystal, the plane along which this change from one twinned direction
to another occurs, can sometimes be visible as a distinct plane in the stone,
sometimes even a slightly reflective one. Corundum (sapphire, ruby) is
particularly prone to this, giving rise to visible striations in the stone The
phenomenon is also responsible for some of the interesting visual effects in
labradorite or moonstone type feldspars, among a number of others. These visible
twinning planes can look like fractures, but are not actual breaks.

Hope that helps.

Peter Rowe

  #4  
Old July 2nd 05, 05:57 PM
norwick
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try
1)
uni of california berkeley: (a real gem of a site)

http://ist-socrates.berkeley.edu/~eps2/wisc/image.html gallery of gems
http://ist-socrates.berkeley.edu/~ep...emologist.html
(ask a gemmologist)

http://ist-socrates.berkeley.edu/~eps2/
(Gems & Gem materials course)

http://ist-socrates.berkeley.edu/~eps2/wisc/jpeg/ index of pics
(lots of goodies - have your pick)

2) if you have access to a good (reference) library
try

Photoatlas of Inclusions in Gemstones
~E.J. Gubelin , J.I. Koivula
(best book on subject)
for info see also
http://www.ganoksin.com/jewelry-book...3855040958.htm

3) try search google images 'inclusions gems'

I hope this helps

regards
norwick


  #5  
Old July 3rd 05, 01:13 AM
Cumberpach
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Thanks to Peter, Carl and norwick for the info. I suspect 'feather curtain'
best describes the flaw in the stone. The ring has gone back for exchange or
refund as there was also a surface chip 1mm x half mm.
I assume the best quality gems get the fancy-cut and the 'swag' gets the
cabochon treatment.

Pete


  #6  
Old July 3rd 05, 01:26 AM
Peter W.. Rowe,
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On Sat, 02 Jul 2005 17:13:19 -0700, in ¸õ "Cumberpach"
wrote:

Thanks to Peter, Carl and norwick for the info. I suspect 'feather curtain'
best describes the flaw in the stone. The ring has gone back for exchange or
refund as there was also a surface chip 1mm x half mm.
I assume the best quality gems get the fancy-cut and the 'swag' gets the
cabochon treatment.

Pete


Swag isn't quite the word I'd use, since the lower clarity grades of amethyst can
still be very pretty, just less expensive. You're right that the very clean
stones are more commonly reserved for faceted cuts, where larger inclusions would
mess up the optics of the stone. Cabochons generally don't need exceptional
clarity to take advantage of optics, since there isn't generally an issue of light
needing to enter the stone and reflect back out unimpeded. In fact, some
inclusions can simply make the stone look more individual and unique, if you can
choose the approach the issue with that mind set, instead of simply lumping it all
in the "flaw" catagory. Some inclusions can be quite fascinating and beautiful.

The simple truth is that inclusions, or flaws, or whatever you wish to call them,
are part of the way mother nature makes gems. They are the signature and
fingerprint of the formation process, and if you crank the magnification up high
enough, there's virtually always something in any stone, somewhere. But of course
it's your money, and if you prefer the look of a very clean stone (and many people
do), then you should not settle for less than what you wish.

But do understand that many commercially produced lines of jewelry using such
cabochon cut inexpensive stones are priced on the assumption that the stones will
not be totally clean. Insisting on a clean one may in some cases be asking for a
ring better than what the seller is normally selling, or has priced the item at.
With amethyst, for example, there can be a significant difference in cost between
a parcel of stones that are eye clean, versus a parcel of the same color that have
visible veils and inclusions, since the latter are far more common with amethyst
cabs. Be prepared, in such cases, to pay the difference in cost between the
stock quality, and the higher clarity one, if that is the case with your seller.

This issue is, by the way, one big disadvantage to buying over the net or via mail
order, as opposed to in person in a traditional store, where you can carefully
examine the stones before buying. On the net or from a catalog, one often is
seeing a photo of a generic example of a piece, rather than the actual one, and
even if it's the actual piece, gems are hard to photograph in such a way that the
inclusions show up realistically. Plus, photographers understandably don't go out
of their way to make inclusions more visible than needed when doing product
photography...

Peter
  #7  
Old July 3rd 05, 04:59 AM
Carl 1 Lucky Texan
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Though often lower cost goods also get rough treatment and might be
thrown into a tray or other container to bang against each other, I feel
the surface chip is the major reason I'd be inclined to return/exchange
the item. As Peter implied, a cab of amethyst with inclusions and
fractures (that do not otherwise put the stone at increased risk of poor
durability) can still be beatifully colored and unique in appearance.

If you have the inclination, joining a local Gem & Mineral club may be
interesting to you. You can learn to cut and polish your own stones and
have them set or learn metalsmithing and set your own stone. Soemtimes
there are colleges offering such courses as well.

Carl


Cumberpach wrote:
Thanks to Peter, Carl and norwick for the info. I suspect 'feather curtain'
best describes the flaw in the stone. The ring has gone back for exchange or
refund as there was also a surface chip 1mm x half mm.
I assume the best quality gems get the fancy-cut and the 'swag' gets the
cabochon treatment.

Pete



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to reply, change ( .not) to ( .net)

  #8  
Old July 3rd 05, 10:05 AM
Rick Cook
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Default

Peter W.. Rowe, wrote:
On Sat, 02 Jul 2005 17:13:19 -0700, in ¸õ "Cumberpach"
wrote:


Thanks to Peter, Carl and norwick for the info. I suspect 'feather curtain'
best describes the flaw in the stone. The ring has gone back for exchange or
refund as there was also a surface chip 1mm x half mm.
I assume the best quality gems get the fancy-cut and the 'swag' gets the
cabochon treatment.

Pete



Swag isn't quite the word I'd use, since the lower clarity grades of amethyst can
still be very pretty, just less expensive. You're right that the very clean
stones are more commonly reserved for faceted cuts, where larger inclusions would
mess up the optics of the stone. Cabochons generally don't need exceptional
clarity to take advantage of optics, since there isn't generally an issue of light
needing to enter the stone and reflect back out unimpeded. In fact, some
inclusions can simply make the stone look more individual and unique, if you can
choose the approach the issue with that mind set, instead of simply lumping it all
in the "flaw" catagory. Some inclusions can be quite fascinating and beautiful.

The simple truth is that inclusions, or flaws, or whatever you wish to call them,
are part of the way mother nature makes gems. They are the signature and
fingerprint of the formation process, and if you crank the magnification up high
enough, there's virtually always something in any stone, somewhere. But of course
it's your money, and if you prefer the look of a very clean stone (and many people
do), then you should not settle for less than what you wish.

But do understand that many commercially produced lines of jewelry using such
cabochon cut inexpensive stones are priced on the assumption that the stones will
not be totally clean. Insisting on a clean one may in some cases be asking for a
ring better than what the seller is normally selling, or has priced the item at.
With amethyst, for example, there can be a significant difference in cost between
a parcel of stones that are eye clean, versus a parcel of the same color that have
visible veils and inclusions, since the latter are far more common with amethyst
cabs. Be prepared, in such cases, to pay the difference in cost between the
stock quality, and the higher clarity one, if that is the case with your seller.

This issue is, by the way, one big disadvantage to buying over the net or via mail
order, as opposed to in person in a traditional store, where you can carefully
examine the stones before buying. On the net or from a catalog, one often is
seeing a photo of a generic example of a piece, rather than the actual one, and
even if it's the actual piece, gems are hard to photograph in such a way that the
inclusions show up realistically. Plus, photographers understandably don't go out
of their way to make inclusions more visible than needed when doing product
photography...

Peter


Personally I enjoy working with included amethyst. You can do beautiful
things cutting free-form cabochons to use the various 'flaws'.

--RC

  #9  
Old July 4th 05, 12:16 AM
Cumberpach
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Posts: n/a
Default


"Carl 1 Lucky Texan" wrote in message

If you have the inclination, joining a local Gem & Mineral club may be
interesting to you. You can learn to cut and polish your own stones and
have them set or learn metalsmithing and set your own stone. Sometimes
there are colleges offering such courses as well.

Where I live, was heavily mined, up till the end of the 19th Cent., tin and
copper chiefly.
There's a disused mine (open to the public) with a very cheap gem shop,
which sparked my interest 3 years ago, when I moved here. I have even found
some low-grade amethyst in the back garden.
If there's no gem club local, there may be one in Plymouth (25 miles East).
Hmm, worth a thought.


  #10  
Old July 4th 05, 12:16 AM
Cumberpach
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Posts: n/a
Default


"Peter W.. Rowe," wrote in message
"Cumberpach" wrote:

Thanks to Peter, Carl and norwick for the info. I suspect 'feather
curtain'
best describes the flaw in the stone. The ring has gone back for exchange
or
refund as there was also a surface chip 1mm x half mm.
I assume the best quality gems get the fancy-cut and the 'swag' gets the
cabochon treatment.

Pete


Swag isn't quite the word I'd use, since the lower clarity grades of
amethyst can
still be very pretty, just less expensive. You're right that the very
clean
stones are more commonly reserved for faceted cuts, where larger
inclusions would


I should have mentioned the fact that I purchased the item on one of those
falling price tv channels.
The ring shown by the presenter and the ring in their light-box were of very
high quality.
They have a 10 day money back policy and they are trying to exchange it, so
I can't complain too much - it's just the hassle factor.


 




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