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 Post subject: Stressed like a tourmaline?
PostPosted: Fri Jan 16, 2009 4:56 am 
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A few months ago I was cutting a green tourmaline and when I polished the crown the stone split into two halves. And one half then showed multiple parallel cracks not present before.

OK, it happens...

But last week I cut a nice round Afghani tourmaline, light green. I had to work around a little fracture in the rough and I had to leave a very little part of it near the girdle, to get a reasonable size. I got a cool tourmaline... but after a little more than 1 day a surprise appeared: a (self censored term) crack just on one side of the table.
Image


I got a similar surprise with another (pink, african) tourmaline. With a much less evident newborn crack.
Both of the cracks are close, but not contiguous, to pre-existent very little cracks. Should the wise cutter cut all the tiny cracks in tourmaline to avoid these nasty surprises? Or it would be just useless?
Not a big deal for me, I'm a cheap guy in buying rough (I'm strictly an amateur newbie cutter and I'm fine with that), but I imagine a person spending big bucks for a beautiful big rough, then you spend time for a proper design, then you cut a beautiful and valuable gem and the day after you discover a stone with a crack under the table, maybe about to split... :evil:

Stress in crystals in interesting, when you are a crystallographer and not a cutter! :P


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PostPosted: Fri Jan 16, 2009 8:46 am 
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Should the wise cutter cut all the tiny cracks in tourmaline to avoid these nasty surprises?


Yes, it is best. I have had similar disappointments, especially with indicolites. Just one of Nature's little practical jokes, the equivalent of an exploding cigar.

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PostPosted: Fri Jan 16, 2009 8:59 am 
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Could a proper thermal annealing work on stressed tourmalines??? :?: :idea:
If you have compositional layers induced stress it shouldn't help too much, if it's lattice defects-induced stress it should...

Edited: It would also change the color, I guess :lol:


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PostPosted: Fri Jan 16, 2009 9:25 am 
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Could a proper thermal annealing work on stressed tourmalines???
If you have compositional layers induced stress it shouldn't help too much, if it's lattice defects-induced stress it should...


I cannot prove it but suspect that because the problem is so frequent in the indicolites it implies a stacking fault or lattice defect, just like chromium not really fitting well into beryl thus making perfect emeralds so rare.

Most people just cut the crazed "rind" off and hope for the best.

Heat treating of tourmalines can be a gamble. One can improve the stone, or just as likely, end up with grey sand. Some treat, some don't. The compositions of tourmalines are so variable that even gold is found in a few of the copper chromophore varieties. And that does not even bond to anything.

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PostPosted: Fri Jan 16, 2009 9:51 am 
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Generally, I agree with Gearloose --- it's best to cut to clean material. But that's isn't always practical in certain roughs.

I've found there are two kinds of feathers in tourmaline: the wavy, irregular feathers that are usually inside of the crystal and those straight cracks that usually start at the surface. I'll always cut out those straight cracks as they will only get larger. I try to cut away the wavy, irregular feathers too, but I don't worry about it if I have to leave them in.

The spontaneous cracking of the stone you describe suggests to me that you either had bad rough (always possible) or were too rough with the rough while cutting. If you use too coarse a grit for preforming, it can induce those straight cracks later in your cutting. I've had a couple that look good all the way to polish, only to see those straight-line cracks of doom appear. Usually I can avoid this by being gentler in the preform stage (e.g., I don't use anything coarser than my 600 sintered if the crystal looks inclined to cracking)

Oh, keep in mind tourmaline is sensitive to heat shock. Too rapid of a temperature change will induce those straight line cracks.


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PostPosted: Fri Jan 16, 2009 10:08 am 
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Yes, the tourmaline xx structure can collect many strange cations... it has quite large channels where you can fit a lot of weird stuff. The growth solution is also extremely rich in cations that wouldn't easily fit in common crystal structures, so... :)
I'm wondering if trace elements can lead to such high stresses. I don't think e.g. the gold amount could be so high to give high stresses. I guess "structural" elements could have a role in that instead. e.g. if you have a (maybe sharp) border elbaite-dravite in the crystal. That could well be a splitting plane, high energy there... but you have a well-defined chemical (and structural) lattice structure difference.

To get more info it'd be cool to study a slab with some chemical mapping and then do some microdiffraction to get macro- and micro-strain data (and a strain map) on the same slab... but I guess I can't snak into a beamline with my stone and do my analyses just for curiosity :lol: (it's a quite time-consuming analysis too...).

Thermal annealing should help the structure to relax, fixing some lattice defects, so lowering the microstrain. We use it to improve the crystallinity of some standard materials. In tourmaline, killing defects that produces colors, moving some elements, inducing stresses from inclusions, etc. A good idea only in theory I guess :) Also because I don't think defects density can be related to the stress cracking my cool tourmaline. But who knows... :?:

Just thinking loud... it happens quite frequently... :roll:

Tourmaline is an interesting material btw...


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PostPosted: Fri Jan 16, 2009 10:18 am 
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pt wrote:
The spontaneous cracking of the stone you describe suggests to me that you either had bad rough (always possible)


It wasn't very good rough for sure. It's the time-bomb issue that is interesting. That pale green above cracked (conchoidal fracture) 1 day after the stone was cut.

pt wrote:
or were too rough with the rough while cutting.


Worn 600 and 1200, + Alumina polishing on a Lighting Lap. It is a small stone.

I forgot another issue I had with the splitted dark green tourmaline: during the polishing stages I had a couple of extremely tiny fragments detaching from the stone keel, but they were big enough to get deep grooves on the facets. So I had to start all over. Then when almost finished the stone broke :P
That stone was cursed, but I find the time-bomb stress more interesting...

btw, thanks for the answers/suggestions


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PostPosted: Fri Jan 16, 2009 10:57 am 
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I usually don't jump into this thread to often but I am glad I did. This is a very good post. I learned a lot from reading what you guys have written. thanks

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PostPosted: Fri Jan 16, 2009 11:12 am 
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amethystguy wrote:
I usually don't jump into this thread to often but I am glad I did. This is a very good post. I learned a lot from reading what you guys have written. thanks


I was just thinking the exact same thing before I got down as far as amethystguy's post.

I'm a goldsmith a bench jeweler, and a gemologist, but you guys amaze me. I think I need to start reading these threads a little closer. Image

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PostPosted: Fri Jan 16, 2009 11:32 am 
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Maybe a thin section of a stressed tourmaline could give some hints under crossed polars... :smt012


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PostPosted: Fri Jan 16, 2009 3:02 pm 
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I posted this message to the USFG list a few years ago;

well tourmaline is undoubtedly the most 'magic'
gemstone we have, science has been slowly explaining some of it's
properties;

M. Louis Lemery, in 1717 concluded that tourmaline was magnetic and that
the magnetism could be induced.

Franz Ulrich Theodor Aepinus, concluded in 1756 "that not only friction
but heat also should develop the electric energy, both positive and
negative, of the tourmaline"(sic).

Since 1921 when piezoelectric minerals were first explored, tourmaline,
which is strongly piezoelectric has been used in gauges to measure
transient blast pressures.

We know that when piezoelectric material is distorted an electric and/or
magnetic charge is produced and vice versa piezoelectric material distorts
when an electrical and/or magnetic charge is applied or induced.

With tourmaline we know that distortion is not the only means of producing
a magnetic and electrical charge, temperature change, friction and
abrasion also do this. The effect however differs in strength and polarity
and of course will cause the crystal to distort according to that strength
and polarity. How much crystal oscillation we have going on at the same
time as cutting or more importantly, polishing will be a significant
factor in the failure rate.

Another source of failure is the rind. I find this term a bit of a misnomer
as we tend to think of a rind as the outer skin of something that is
growing inside. A crystal grows by deposition so the 'rind' is the newest
part of the stone, the core being the oldest. Now we know that the
crystal is formed over long periods of time and we know that it requires
heat and pressure, we also know that the tourmaline crystal will deform in
response to that heat and pressure whereas the new deposition will not be
so stressed.

As the crystal grows there will be unstressed growth covering stressed
growth until the heat and pressure changes and another cycle of growth
occurs. Flawless gems will only occur if the temperatures allow the
crystal to anneal as it grows. This does not necessarily mean that the
last growth was annealed when the stone was mined so there is every
possibility that just below the unstressed newest layer lies a crystal
core that needs to relax.

THE TOURMALINE PROCEDURE

Removal of the outer layer is universally recommended.
Using hand held dops to do this has been strongly suggested.

That all grinding and polishing should be along the length of the crystal
not across it, using fine grit freshly charged laps is universally
recommended.

The crystal has one end designated the head and the other the tail. All
cutting and polishing will be in the same direction. This means you need a
reversable motor to accomplish this. Many say you don't need to do this
but adherents claim 100 percent success rate.

Using prewarmed laps and warm water for cutting and polishing is strongly
recommended.

Heavy flushing during cutting and polishing is also strongly recommended

Accomplishing the entire cutting process as quickly as possible is highly
recommended. Leaving a half completed stone for another day's session is
warned against.

Annealing.
I don't know the source but I believe Farouq assigned it to Marc
Liccini. The suggestion is that brittle tourmaline can be annealed in a
toaster oven. 450F is the temperature that will change the colour or
saturation so it would probably be wise to stay below that. I have never
tried annealing tourmaline but the suggestion is that an annealed stone
has fewer untoward cutting characteristics and doesn't require as much
special care and attention. No experience, no comment.

Tony.

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PostPosted: Fri Jan 16, 2009 6:53 pm 
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To get more info it'd be cool to study a slab with some chemical mapping and then do some microdiffraction to get macro- and micro-strain data (and a strain map) on the same slab...


Sadly I missed my chance to look into this. A few years before I retired, my company sent my EDAX to another division. It would have been perfect and could have been done in a lunch hour, with element dot maps, etc.

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PostPosted: Fri Jan 16, 2009 7:48 pm 
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Tony -- I saw this Tourmaline Procedure when you posted it to the USFG a while back. I have it in my folder of important faceting stuff. The most important tip for me so far has been the cutting & polishing with the length of the crystal. (It seems that if I forget, or sweep too far on the lap, the crystal reminds me in no uncertain terms...)

But I'm not sure how this point of advice is supposed to work:

Quote:
Using prewarmed laps and warm water for cutting and polishing is strongly
recommended.


I tried it the other day and found that the prewarmed lap quickly becomes a cold lap while it's spinning in my 65F office. The water in the drip stays warm, of course, but then I'm left with the problem of warm water on a cold lap and a heat sensitive stone in contact with both. That didn't seem so wise, so I gave up and changed the drip back to room temp water.

I'm always interested in improving the odds with tourmaline. Do you have any insight on this point?

peter

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PostPosted: Fri Jan 16, 2009 10:38 pm 
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I must have cut about 800 or more tourmalines, and to date, have had 4 stones crack as described. One was a very pretty green/blue stone from Afghanistan, and the other 3 were all blue Namibian stones.

I don't use any of the USFG precautions on any of the 800 plus stones. Most are roughed in with a 260 or 360 lap, then finish cut with a 600 nubond. Cold water, room temperature laps, laps spinning in the same direction. I would rather loose 4 out of 800 stones, but save 2 hours per stone in cutting time. That would = maybe 1600 hours of saved time x lets say $30 per hr. = $48,000. I doubt the 4 stones I lost were worth $48,000

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 Post subject: warm laps and water
PostPosted: Sat Jan 17, 2009 12:26 am 
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Hello Peter,

Yeah, I pondered a bit on that one when I was told too. Actually the answer was a bit obvious, room temperature. Apparently some cutters arrive at work to a cold building and use cold tap water. I cut in a heated office and have a room temperature water tank like most of us. Basement and garage cutters have remarked on this however.

Hello Precision,

I hear the worst offenders are indicolite and chrome and the bigger and better ones are the most prone. I lost a couple of rather pricey chrome tourmalines which separated into several pieces about 25 yrs back. I did seek advice back then and I have been collecting suggestions since. I came up with the 'tourmaline procedure' from these suggestions.

I am not guaranteeing that any of these suggestions will definitely prevent any catastrophic event, however they were given to me by cutters that had considerably more tourmaline cutting experience than I had and I'm only passing them along in the hope that they are useful to cutters having problems.

I certainly have no issue with anyone that has success without using any of them but I do suspect that those are only cutters with a lot of experience. I do have a bit of a problem with the extra 2 hours as it makes no difference to my cutting times. I don't anneal my stones though but that is the only suggestion that should prolong the session. I do have a big problem with the $30 per hour, that would hardy cover my office expenses. I charge by the job and never reveal how long it took me. You need a raise, your work is far too good to be giving it away like that. Tut, I say tut tut.

Tony.

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